Trevor Harmon's UIImage categories (http://vocaro.com/trevor/blog/2009/10/12/resize-a-uiimage-the-right-way/) with iOS5 fixes (https://gist.github.com/1344380)

UIImage+Alpha.h

// UIImage+Alpha.h
// Created by Trevor Harmon on 9/20/09.
// Free for personal or commercial use, with or without modification.
// No warranty is expressed or implied.

// Helper methods for adding an alpha layer to an image
@interface UIImage (Alpha)
- (BOOL)hasAlpha;
- (UIImage *)imageWithAlpha;
- (UIImage *)transparentBorderImage:(NSUInteger)borderSize;
@end

UIImage+Alpha.m

// UIImage+Alpha.m
// Created by Trevor Harmon on 9/20/09.
// Free for personal or commercial use, with or without modification.
// No warranty is expressed or implied.

#import "UIImage+Alpha.h"

// Private helper methods
@interface UIImage ()
- (CGImageRef)newBorderMask:(NSUInteger)borderSize size:(CGSize)size;
@end

@implementation UIImage (Alpha)

// Returns true if the image has an alpha layer
- (BOOL)hasAlpha {
    CGImageAlphaInfo alpha = CGImageGetAlphaInfo(self.CGImage);
    return (alpha == kCGImageAlphaFirst ||
            alpha == kCGImageAlphaLast ||
            alpha == kCGImageAlphaPremultipliedFirst ||
            alpha == kCGImageAlphaPremultipliedLast);
}

// Returns a copy of the given image, adding an alpha channel if it doesn't already have one
- (UIImage *)imageWithAlpha {
    if ([self hasAlpha]) {
        return self;
    }
    
    CGImageRef imageRef = self.CGImage;
    size_t width = CGImageGetWidth(imageRef);
    size_t height = CGImageGetHeight(imageRef);
    
    // The bitsPerComponent and bitmapInfo values are hard-coded to prevent an "unsupported parameter combination" error
    CGContextRef offscreenContext = CGBitmapContextCreate(NULL,
                                                          width,
                                                          height,
                                                          8,
                                                          0,
                                                          CGImageGetColorSpace(imageRef),
                                                          kCGBitmapByteOrderDefault | kCGImageAlphaPremultipliedFirst);
    
    // Draw the image into the context and retrieve the new image, which will now have an alpha layer
    CGContextDrawImage(offscreenContext, CGRectMake(0, 0, width, height), imageRef);
    CGImageRef imageRefWithAlpha = CGBitmapContextCreateImage(offscreenContext);
    UIImage *imageWithAlpha = [UIImage imageWithCGImage:imageRefWithAlpha];
    
    // Clean up
    CGContextRelease(offscreenContext);
    CGImageRelease(imageRefWithAlpha);
    
    return imageWithAlpha;
}

// Returns a copy of the image with a transparent border of the given size added around its edges.
// If the image has no alpha layer, one will be added to it.
- (UIImage *)transparentBorderImage:(NSUInteger)borderSize {
    // If the image does not have an alpha layer, add one
    UIImage *image = [self imageWithAlpha];
    
    CGRect newRect = CGRectMake(0, 0, image.size.width + borderSize * 2, image.size.height + borderSize * 2);
    
    // Build a context that's the same dimensions as the new size
    CGContextRef bitmap = CGBitmapContextCreate(NULL,
                                                newRect.size.width,
                                                newRect.size.height,
                                                CGImageGetBitsPerComponent(self.CGImage),
                                                0,
                                                CGImageGetColorSpace(self.CGImage),
                                                CGImageGetBitmapInfo(self.CGImage));
    
    // Draw the image in the center of the context, leaving a gap around the edges
    CGRect imageLocation = CGRectMake(borderSize, borderSize, image.size.width, image.size.height);
    CGContextDrawImage(bitmap, imageLocation, self.CGImage);
    CGImageRef borderImageRef = CGBitmapContextCreateImage(bitmap);
    
    // Create a mask to make the border transparent, and combine it with the image
    CGImageRef maskImageRef = [self newBorderMask:borderSize size:newRect.size];
    CGImageRef transparentBorderImageRef = CGImageCreateWithMask(borderImageRef, maskImageRef);
    UIImage *transparentBorderImage = [UIImage imageWithCGImage:transparentBorderImageRef];
    
    // Clean up
    CGContextRelease(bitmap);
    CGImageRelease(borderImageRef);
    CGImageRelease(maskImageRef);
    CGImageRelease(transparentBorderImageRef);
    
    return transparentBorderImage;
}

#pragma mark -
#pragma mark Private helper methods

// Creates a mask that makes the outer edges transparent and everything else opaque
// The size must include the entire mask (opaque part + transparent border)
// The caller is responsible for releasing the returned reference by calling CGImageRelease
- (CGImageRef)newBorderMask:(NSUInteger)borderSize size:(CGSize)size {
    CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceGray();
    
    // Build a context that's the same dimensions as the new size
    CGContextRef maskContext = CGBitmapContextCreate(NULL,
                                                     size.width,
                                                     size.height,
                                                     8, // 8-bit grayscale
                                                     0,
                                                     colorSpace,
                                                     kCGBitmapByteOrderDefault | kCGImageAlphaNone);
    
    // Start with a mask that's entirely transparent
    CGContextSetFillColorWithColor(maskContext, [UIColor blackColor].CGColor);
    CGContextFillRect(maskContext, CGRectMake(0, 0, size.width, size.height));
    
    // Make the inner part (within the border) opaque
    CGContextSetFillColorWithColor(maskContext, [UIColor whiteColor].CGColor);
    CGContextFillRect(maskContext, CGRectMake(borderSize, borderSize, size.width - borderSize * 2, size.height - borderSize * 2));
    
    // Get an image of the context
    CGImageRef maskImageRef = CGBitmapContextCreateImage(maskContext);
    
    // Clean up
    CGContextRelease(maskContext);
    CGColorSpaceRelease(colorSpace);
    
    return maskImageRef;
}

@end

UIImage+Resize.h

// UIImage+Resize.h
// Created by Trevor Harmon on 8/5/09.
// Free for personal or commercial use, with or without modification.
// No warranty is expressed or implied.

// Extends the UIImage class to support resizing/cropping
@interface UIImage (Resize)
- (UIImage *)croppedImage:(CGRect)bounds;
- (UIImage *)thumbnailImage:(NSInteger)thumbnailSize
          transparentBorder:(NSUInteger)borderSize
               cornerRadius:(NSUInteger)cornerRadius
       interpolationQuality:(CGInterpolationQuality)quality;
- (UIImage *)resizedImage:(CGSize)newSize
     interpolationQuality:(CGInterpolationQuality)quality;
- (UIImage *)resizedImageWithContentMode:(UIViewContentMode)contentMode
                                  bounds:(CGSize)bounds
                    interpolationQuality:(CGInterpolationQuality)quality;
@end

UIImage+Resize.m

// UIImage+Resize.m
// Created by Trevor Harmon on 8/5/09.
// Free for personal or commercial use, with or without modification.
// No warranty is expressed or implied.

#import "UIImage+Resize.h"
#import "UIImage+RoundedCorner.h"
#import "UIImage+Alpha.h"

// Private helper methods
@interface UIImage ()
- (UIImage *)resizedImage:(CGSize)newSize
                transform:(CGAffineTransform)transform
           drawTransposed:(BOOL)transpose
     interpolationQuality:(CGInterpolationQuality)quality;
- (CGAffineTransform)transformForOrientation:(CGSize)newSize;
@end

@implementation UIImage (Resize)

// Returns a copy of this image that is cropped to the given bounds.
// The bounds will be adjusted using CGRectIntegral.
// This method ignores the image's imageOrientation setting.
- (UIImage *)croppedImage:(CGRect)bounds {
    CGImageRef imageRef = CGImageCreateWithImageInRect([self CGImage], bounds);
    UIImage *croppedImage = [UIImage imageWithCGImage:imageRef];
    CGImageRelease(imageRef);
    return croppedImage;
}

// Returns a copy of this image that is squared to the thumbnail size.
// If transparentBorder is non-zero, a transparent border of the given size will be added around the edges of the thumbnail. (Adding a transparent border of at least one pixel in size has the side-effect of antialiasing the edges of the image when rotating it using Core Animation.)
- (UIImage *)thumbnailImage:(NSInteger)thumbnailSize
          transparentBorder:(NSUInteger)borderSize
               cornerRadius:(NSUInteger)cornerRadius
       interpolationQuality:(CGInterpolationQuality)quality {
    UIImage *resizedImage = [self resizedImageWithContentMode:UIViewContentModeScaleAspectFill
                                                       bounds:CGSizeMake(thumbnailSize, thumbnailSize)
                                         interpolationQuality:quality];
    
    // Crop out any part of the image that's larger than the thumbnail size
    // The cropped rect must be centered on the resized image
    // Round the origin points so that the size isn't altered when CGRectIntegral is later invoked
    CGRect cropRect = CGRectMake(round((resizedImage.size.width - thumbnailSize) / 2),
                                 round((resizedImage.size.height - thumbnailSize) / 2),
                                 thumbnailSize,
                                 thumbnailSize);
    UIImage *croppedImage = [resizedImage croppedImage:cropRect];
    
    UIImage *transparentBorderImage = borderSize ? [croppedImage transparentBorderImage:borderSize] : croppedImage;

    return [transparentBorderImage roundedCornerImage:cornerRadius borderSize:borderSize];
}

// Returns a rescaled copy of the image, taking into account its orientation
// The image will be scaled disproportionately if necessary to fit the bounds specified by the parameter
- (UIImage *)resizedImage:(CGSize)newSize interpolationQuality:(CGInterpolationQuality)quality {
    BOOL drawTransposed;
    
    switch (self.imageOrientation) {
        case UIImageOrientationLeft:
        case UIImageOrientationLeftMirrored:
        case UIImageOrientationRight:
        case UIImageOrientationRightMirrored:
            drawTransposed = YES;
            break;
            
        default:
            drawTransposed = NO;
    }
    
    return [self resizedImage:newSize
                    transform:[self transformForOrientation:newSize]
               drawTransposed:drawTransposed
         interpolationQuality:quality];
}

// Resizes the image according to the given content mode, taking into account the image's orientation
- (UIImage *)resizedImageWithContentMode:(UIViewContentMode)contentMode
                                  bounds:(CGSize)bounds
                    interpolationQuality:(CGInterpolationQuality)quality {
    CGFloat horizontalRatio = bounds.width / self.size.width;
    CGFloat verticalRatio = bounds.height / self.size.height;
    CGFloat ratio;
    
    switch (contentMode) {
        case UIViewContentModeScaleAspectFill:
            ratio = MAX(horizontalRatio, verticalRatio);
            break;
            
        case UIViewContentModeScaleAspectFit:
            ratio = MIN(horizontalRatio, verticalRatio);
            break;
            
        default:
            [NSException raise:NSInvalidArgumentException format:@"Unsupported content mode: %d", contentMode];
    }
    
    CGSize newSize = CGSizeMake(self.size.width * ratio, self.size.height * ratio);
    
    return [self resizedImage:newSize interpolationQuality:quality];
}

#pragma mark -
#pragma mark Private helper methods

// Returns a copy of the image that has been transformed using the given affine transform and scaled to the new size
// The new image's orientation will be UIImageOrientationUp, regardless of the current image's orientation
// If the new size is not integral, it will be rounded up
- (UIImage *)resizedImage:(CGSize)newSize
                transform:(CGAffineTransform)transform
           drawTransposed:(BOOL)transpose
     interpolationQuality:(CGInterpolationQuality)quality {
    CGRect newRect = CGRectIntegral(CGRectMake(0, 0, newSize.width, newSize.height));
    CGRect transposedRect = CGRectMake(0, 0, newRect.size.height, newRect.size.width);
    CGImageRef imageRef = self.CGImage;
    
    // Build a context that's the same dimensions as the new size
    CGContextRef bitmap = CGBitmapContextCreate(NULL,
                                                newRect.size.width,
                                                newRect.size.height,
                                                CGImageGetBitsPerComponent(imageRef),
                                                0,
                                                CGImageGetColorSpace(imageRef),
                                                CGImageGetBitmapInfo(imageRef));
    
    // Rotate and/or flip the image if required by its orientation
    CGContextConcatCTM(bitmap, transform);
    
    // Set the quality level to use when rescaling
    CGContextSetInterpolationQuality(bitmap, quality);
    
    // Draw into the context; this scales the image
    CGContextDrawImage(bitmap, transpose ? transposedRect : newRect, imageRef);
    
    // Get the resized image from the context and a UIImage
    CGImageRef newImageRef = CGBitmapContextCreateImage(bitmap);
    UIImage *newImage = [UIImage imageWithCGImage:newImageRef];
    
    // Clean up
    CGContextRelease(bitmap);
    CGImageRelease(newImageRef);
    
    return newImage;
}

// Returns an affine transform that takes into account the image orientation when drawing a scaled image
- (CGAffineTransform)transformForOrientation:(CGSize)newSize {
    CGAffineTransform transform = CGAffineTransformIdentity;
    
    switch (self.imageOrientation) {
        case UIImageOrientationDown:           // EXIF = 3
        case UIImageOrientationDownMirrored:   // EXIF = 4
            transform = CGAffineTransformTranslate(transform, newSize.width, newSize.height);
            transform = CGAffineTransformRotate(transform, M_PI);
            break;
            
        case UIImageOrientationLeft:           // EXIF = 6
        case UIImageOrientationLeftMirrored:   // EXIF = 5
            transform = CGAffineTransformTranslate(transform, newSize.width, 0);
            transform = CGAffineTransformRotate(transform, M_PI_2);
            break;
            
        case UIImageOrientationRight:          // EXIF = 8
        case UIImageOrientationRightMirrored:  // EXIF = 7
            transform = CGAffineTransformTranslate(transform, 0, newSize.height);
            transform = CGAffineTransformRotate(transform, -M_PI_2);
            break;
    }
    
    switch (self.imageOrientation) {
        case UIImageOrientationUpMirrored:     // EXIF = 2
        case UIImageOrientationDownMirrored:   // EXIF = 4
            transform = CGAffineTransformTranslate(transform, newSize.width, 0);
            transform = CGAffineTransformScale(transform, -1, 1);
            break;
            
        case UIImageOrientationLeftMirrored:   // EXIF = 5
        case UIImageOrientationRightMirrored:  // EXIF = 7
            transform = CGAffineTransformTranslate(transform, newSize.height, 0);
            transform = CGAffineTransformScale(transform, -1, 1);
            break;
    }
    
    return transform;
}

@end

UIImage+RoundedCorner.h

// UIImage+RoundedCorner.h
// Created by Trevor Harmon on 9/20/09.
// Free for personal or commercial use, with or without modification.
// No warranty is expressed or implied.

// Extends the UIImage class to support making rounded corners
@interface UIImage (RoundedCorner)
- (UIImage *)roundedCornerImage:(NSInteger)cornerSize borderSize:(NSInteger)borderSize;
@end

UIImage+RoundedCorner.m

// UIImage+RoundedCorner.m
// Created by Trevor Harmon on 9/20/09.
// Free for personal or commercial use, with or without modification.
// No warranty is expressed or implied.

#import "UIImage+RoundedCorner.h"
#import "UIImage+Alpha.h"

// Private helper methods
@interface UIImage ()
- (void)addRoundedRectToPath:(CGRect)rect context:(CGContextRef)context ovalWidth:(CGFloat)ovalWidth ovalHeight:(CGFloat)ovalHeight;
@end

@implementation UIImage (RoundedCorner)

// Creates a copy of this image with rounded corners
// If borderSize is non-zero, a transparent border of the given size will also be added
// Original author: Björn Sållarp. Used with permission. See: http://blog.sallarp.com/iphone-uiimage-round-corners/
- (UIImage *)roundedCornerImage:(NSInteger)cornerSize borderSize:(NSInteger)borderSize {
    // If the image does not have an alpha layer, add one
    UIImage *image = [self imageWithAlpha];
    
    // Build a context that's the same dimensions as the new size
    CGContextRef context = CGBitmapContextCreate(NULL,
                                                 image.size.width,
                                                 image.size.height,
                                                 CGImageGetBitsPerComponent(image.CGImage),
                                                 0,
                                                 CGImageGetColorSpace(image.CGImage),
                                                 CGImageGetBitmapInfo(image.CGImage));

    // Create a clipping path with rounded corners
    CGContextBeginPath(context);
    [self addRoundedRectToPath:CGRectMake(borderSize, borderSize, image.size.width - borderSize * 2, image.size.height - borderSize * 2)
                       context:context
                     ovalWidth:cornerSize
                    ovalHeight:cornerSize];
    CGContextClosePath(context);
    CGContextClip(context);

    // Draw the image to the context; the clipping path will make anything outside the rounded rect transparent
    CGContextDrawImage(context, CGRectMake(0, 0, image.size.width, image.size.height), image.CGImage);
    
    // Create a CGImage from the context
    CGImageRef clippedImage = CGBitmapContextCreateImage(context);
    CGContextRelease(context);
    
    // Create a UIImage from the CGImage
    UIImage *roundedImage = [UIImage imageWithCGImage:clippedImage];
    CGImageRelease(clippedImage);
    
    return roundedImage;
}

#pragma mark -
#pragma mark Private helper methods

// Adds a rectangular path to the given context and rounds its corners by the given extents
// Original author: Björn Sållarp. Used with permission. See: http://blog.sallarp.com/iphone-uiimage-round-corners/
- (void)addRoundedRectToPath:(CGRect)rect context:(CGContextRef)context ovalWidth:(CGFloat)ovalWidth ovalHeight:(CGFloat)ovalHeight {
    if (ovalWidth == 0 || ovalHeight == 0) {
        CGContextAddRect(context, rect);
        return;
    }
    CGContextSaveGState(context);
    CGContextTranslateCTM(context, CGRectGetMinX(rect), CGRectGetMinY(rect));
    CGContextScaleCTM(context, ovalWidth, ovalHeight);
    CGFloat fw = CGRectGetWidth(rect) / ovalWidth;
    CGFloat fh = CGRectGetHeight(rect) / ovalHeight;
    CGContextMoveToPoint(context, fw, fh/2);
    CGContextAddArcToPoint(context, fw, fh, fw/2, fh, 1);
    CGContextAddArcToPoint(context, 0, fh, 0, fh/2, 1);
    CGContextAddArcToPoint(context, 0, 0, fw/2, 0, 1);
    CGContextAddArcToPoint(context, fw, 0, fw, fh/2, 1);
    CGContextClosePath(context);
    CGContextRestoreGState(context);
}

@end

is this still working for ios5.1, do you have a usage example?

- (UIImage *)thumbnailImage:(NSInteger)thumbnailSize
          transparentBorder:(NSUInteger)borderSize
               cornerRadius:(NSUInteger)cornerRadius
       interpolationQuality:(CGInterpolationQuality)quality; 

Hi Ryan, this works under 5.1 as far as i know, although I'm only using the 'resizedImage' method.

looks like it is, thx

hello, awesome, could you make the swift version ?