+ (UIImage *)handleImage:(UIImage *)originalImage withSize:(CGSize)size
{
    CGSize originalsize = [originalImage size];
    NSLog(@"改变前图片的宽度为%f,图片的高度为%f",originalsize.width,originalsize.height);
     
    //原图长宽均小于标准长宽的，不作处理返回原图
    if (originalsize.width<size.width && originalsize.height<size.height)
    {
        return originalImage;
    }
     
    //原图长宽均大于标准长宽的，按比例缩小至最大适应值
    else if(originalsize.width>size.width && originalsize.height>size.height)
    {
        CGFloat rate = 1.0;
        CGFloat widthRate = originalsize.width/size.width;
        CGFloat heightRate = originalsize.height/size.height;
         
        rate = widthRate>heightRate?heightRate:widthRate;
         
        CGImageRef imageRef = nil;
         
        if (heightRate>widthRate)
        {
            imageRef = CGImageCreateWithImageInRect([originalImage CGImage], CGRectMake(0, originalsize.height/2-size.height*rate/2, originalsize.width, size.height*rate));//获取图片整体部分
        }
        else
        {
            imageRef = CGImageCreateWithImageInRect([originalImage CGImage], CGRectMake(originalsize.width/2-size.width*rate/2, 0, size.width*rate, originalsize.height));//获取图片整体部分
        }
        UIGraphicsBeginImageContext(size);//指定要绘画图片的大小
        CGContextRef con = UIGraphicsGetCurrentContext();
         
        CGContextTranslateCTM(con, 0.0, size.height);
        CGContextScaleCTM(con, 1.0, -1.0);
         
        CGContextDrawImage(con, CGRectMake(0, 0, size.width, size.height), imageRef);
         
        UIImage *standardImage = UIGraphicsGetImageFromCurrentImageContext();
        NSLog(@"改变后图片的宽度为%f,图片的高度为%f",[standardImage size].width,[standardImage size].height);
         
        UIGraphicsEndImageContext();
        CGImageRelease(imageRef);
         
        return standardImage;
    }
     
    //原图长宽有一项大于标准长宽的，对大于标准的那一项进行裁剪，另一项保持不变
    else if(originalsize.height>size.height || originalsize.width>size.width)
    {
        CGImageRef imageRef = nil;
         
        if(originalsize.height>size.height)
        {
            imageRef = CGImageCreateWithImageInRect([originalImage CGImage], CGRectMake(0, originalsize.height/2-size.height/2, originalsize.width, size.height));//获取图片整体部分
        }
        else if (originalsize.width>size.width)
        {
            imageRef = CGImageCreateWithImageInRect([originalImage CGImage], CGRectMake(originalsize.width/2-size.width/2, 0, size.width, originalsize.height));//获取图片整体部分
        }
         
        UIGraphicsBeginImageContext(size);//指定要绘画图片的大小
        CGContextRef con = UIGraphicsGetCurrentContext();
         
        CGContextTranslateCTM(con, 0.0, size.height);
        CGContextScaleCTM(con, 1.0, -1.0);
         
        CGContextDrawImage(con, CGRectMake(0, 0, size.width, size.height), imageRef);
         
        UIImage *standardImage = UIGraphicsGetImageFromCurrentImageContext();
        NSLog(@"改变后图片的宽度为%f,图片的高度为%f",[standardImage size].width,[standardImage size].height);
         
        UIGraphicsEndImageContext();
        CGImageRelease(imageRef);
         
        return standardImage;
    }
     
    //原图为标准长宽的，不做处理
    else
    {
        return originalImage;
    }
}

原文链接