アスペクト比（縦横比）を保ったまま、画像を拡大縮小する。

iphone - How to scale a UIImageView proportionally? - Stack Overflow

@interface UIImage (Extras)
- (UIImage *)imageByScalingProportionallyToSize:(CGSize)targetSize;
@end;

@implementation UIImage (Extras)

- (UIImage *)imageByScalingProportionallyToSize:(CGSize)targetSize {
    UIImage *sourceImage = self;
    UIImage *newImage = nil;
    
    CGSize imageSize = sourceImage.size;
    CGFloat width = imageSize.width;
    CGFloat height = imageSize.height;
    
    CGFloat targetWidth = targetSize.width;
    CGFloat targetHeight = targetSize.height;
    
    CGFloat scaleFactor = 0.0f;
    CGFloat scaledWidth = targetWidth;
    CGFloat scaledHeight = targetHeight;
    
    CGPoint thumbnailPoint = CGPointMake(0.0f, 0.0f);
    
    if (CGSizeEqualToSize(imageSize, targetSize) == NO) {
        CGFloat widthFactor = targetWidth / width;
        CGFloat heightFactor = targetHeight / height;
        
        if (widthFactor < heightFactor) {
            scaleFactor = widthFactor;
        } else {
            scaleFactor = heightFactor;
        }
        
        scaledWidth  = width * scaleFactor;
        scaledHeight = height * scaleFactor;
        
        // center the image
        if (widthFactor < heightFactor) {
            thumbnailPoint.y = (targetHeight - scaledHeight) * 0.5f; 
        } else if (widthFactor > heightFactor) {
            thumbnailPoint.x = (targetWidth - scaledWidth) * 0.5f;
        }
    }
    
    // this is actually the interesting part:
    UIGraphicsBeginImageContext(targetSize);
    
    [[UIColor blackColor] set];
    CGContextRef context = UIGraphicsGetCurrentContext();
    CGContextFillRect(context, CGRectMake(0.0f, 0.0f, targetWidth, targetHeight));
    
    CGRect thumbnailRect = CGRectZero;
    thumbnailRect.origin = thumbnailPoint;
    thumbnailRect.size.width  = scaledWidth;
    thumbnailRect.size.height = scaledHeight;
    
    [sourceImage drawInRect:thumbnailRect];
    
    newImage = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    
    if (newImage == nil) NSLog(@"could not scale image");
    
    return newImage ;
}

@end