uses CGImageCreateWithImageInRect to perform a crop

gistfile1.m

    // this chunk of code loads a jpeg image into a cgimage
    // creates a second crop of the original image with CGImageCreateWithImageInRect
    // writes the new cropped image to the desktop
    // ensure that the xy origin of the CGRectMake call is smaller than the width or height of the original image
    
    NSURL *originalImage = [NSURL fileURLWithPath:[[NSBundle mainBundle] pathForResource:@"lockwood" ofType:@"jpg"]];
    CGImageRef imageRef = NULL;
    
    CGImageSourceRef loadRef = CGImageSourceCreateWithURL((CFURLRef)originalImage, NULL);
    if (loadRef != NULL)
    {
        imageRef = CGImageSourceCreateImageAtIndex(loadRef, 0, NULL);
        CFRelease(loadRef); // Release CGImageSource reference
    }    
    CGImageRef croppedImage = CGImageCreateWithImageInRect(imageRef, CGRectMake(200., 200., 100., 100.));
        
    CFURLRef saveUrl = (CFURLRef)[NSURL fileURLWithPath:[@"~/Desktop/lockwood-crop.jpg" stringByExpandingTildeInPath]];
    CGImageDestinationRef destination = CGImageDestinationCreateWithURL(saveUrl, kUTTypeJPEG, 1, NULL);
    CGImageDestinationAddImage(destination, croppedImage, nil);
    
    if (!CGImageDestinationFinalize(destination)) {
        NSLog(@"Failed to write image to %@", saveUrl);
    }
    
    CFRelease(destination);
    CFRelease(imageRef);
    CFRelease(croppedImage);