这方法挺实用的，直接调用就可以得到想要的size。
//将一个UIImage缩放变换到指定Size
-(UIImage*) OriginImage:(UIImage *)image scaleToSize:(CGSize)size
{
UIGraphicsBeginImageContext(size);
[image drawInRect:CGRectMake(0, 0, size.width, size.height)];
UIImage* scaledImage = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
return scaledImage;
}