Using 16 bits instead of 32 bits cuts memory usage in half : http://www.dwellable.com/blog/Tech-iOS-Image-Tricks

gIsLowCPU_gIsLowRam.m

//
// gModel and friends
//
// iphone
// 1:   412mhz 128mb iPhone1,1
// 3g:  412mhz 128mb iPhone1,2
// 3gs: 600mhz 256mb iPhone2,1
// 4:   800mhz 512mb iPhone3,1-3
// 4s:  2x1ghz 512mb iPhone4,1
//    
// ipad
// 1:   1ghz 256mb iPad1,1-2
// 2: 2x1ghz 512mb iPad2,1-2
// 3: 2x1ghz 1gb   iPad3
//    
// ipod
// 1: 412mhz 128mb iPod1,1
// 2: 533mhz 128mb iPod2,1
// 3: 600mhz 256mb iPod3,1
// 4: 800mhz 256mb iPod4,1
//

struct utsname u;
uname(&u);
gModel = [NSString stringWithCString:u.machine encoding:NSASCIIStringEncoding];

// < 600mhz
gIsLowCPU = [gModel matches:[NSRegularExpression re:@"^(iPhone1|iPod[12])"]];
// old iphones/ipod/ipads
gIsLowRam = [gModel matches:[NSRegularExpression re:@"^(iPhone1|iPod[12]|iPad1)"]];


//
// For anybody trying to copy 'n paste:
//

@implementation NSRegularExpression (Dwellable)

+ (NSRegularExpression *)re:(NSString *)s
{
    return[NSRegularExpression regularExpressionWithPattern:s options:0 error:nil];
}

- (BOOL)matches:(NSString *)s
{
    return [self numberOfMatchesInString:s options:0 range:NSMakeRange(0, s.length)];
}

@end

@implementation NSString (Dwellable)

- (BOOL)matches:(NSRegularExpression *)re
{
    return [re matches:self];
}

@end

UIImage+Lazy.m

@implementation UIImage (Dwellable)

- (UIImage *)crop:(CGRect)rect
{
  return [self copyFromRect:rect toSize:rect.size];
}

- (UIImage *)copyToSize:(CGSize)dstSize
{
  return [self copyFromRect:CGRectMake(0, 0, self.size.width, self.size.height)
               toSize:dstSize];
}

- (UIImage *)copyFromRect:(CGRect)srcRect toSize:(CGSize)dstSize
{
  //
  // apply scale if necessary
  //

  int scale = self.scale;
  if (scale > 1) {
    srcRect = CGRectMake(srcRect.origin.x   * scale, srcRect.origin.y    * scale,
                         srcRect.size.width * scale, srcRect.size.height * scale);
    dstSize = CGSizeMake(dstSize.width * scale, dstSize.height * scale);
  }

  //
  // create the context
  //

  int bpc = 8;
  CGBitmapInfo bmi = kCGImageAlphaPremultipliedFirst;
  CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
  if (gIsLowRam) {
    // 16bit if we don't have an alpha channel (this saves a ton of memory)
    CGImageAlphaInfo alpha = CGImageGetAlphaInfo(self.CGImage);
    if (alpha == kCGImageAlphaNone || alpha == kCGImageAlphaNoneSkipLast ||
        alpha == kCGImageAlphaNoneSkipFirst) {
      bpc = 5;
      bmi = kCGImageAlphaNoneSkipFirst;
    }
  }
  CGContextRef context = CGBitmapContextCreate(NULL, dstSize.width, dstSize.height,
                                               bpc, 0, colorSpace, bmi);
  CGColorSpaceRelease(colorSpace);

  //
  // interpolation quality set to high on everything but old iphones
  //

  CGInterpolationQuality quality = kCGInterpolationHigh;
  if (gIsLowCPU) {
    quality = kCGInterpolationDefault;
  }
  CGContextSetInterpolationQuality(context, quality);

  //
  // choose srcImage
  //

  CGImageRef srcImage;
  if (!CGRectEqualToRect(srcRect, CGRectMake(0, 0, self.size.width * scale,
                                             self.size.height * scale))) {
    srcImage = CGImageCreateWithImageInRect(self.CGImage, srcRect);
  } else {
    srcImage = CGImageRetain(self.CGImage);
  }

  //
  // copy
  //

  CGContextDrawImage(context, CGRectMake(0, 0, dstSize.width, dstSize.height),
                     srcImage);
  CGImageRelease(srcImage);
  CGImageRef dst = CGBitmapContextCreateImage(context);
  CGContextRelease(context);

  UIImage *result = [UIImage imageWithCGImage:dst scale:scale
                             orientation:UIImageOrientationUp];
  CGImageRelease(dst);  
  return result;
}