Some functions are like recurring nightmares... I have this vague feeling I've written this many times before.

gistfile1.m

UIImage* aspectFitImage(UIImage* source, CGSize size)
{
    const float wr = source.size.width/size.width;
    const float hr = source.size.height/size.height;
    const float scale = 1.0f/MIN(wr, hr);
    const float width = ceilf(source.size.width*scale);
    const float height = ceilf(source.size.height*scale);
    CGRect rect = {(size.width-width)*0.5f, (size.height-height)*0.5f, width, height};
    
    UIGraphicsBeginImageContextWithOptions(size, NO, 0.0f);
    [source drawInRect:rect];
    UIImage* dest = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    
    return dest;
}