使用下载选项 SDWebImageAvoidAutoSetImage， 下载完成后在后台剪裁，然后在主线程设置image并调用刷新。
可以给 UIImageView 写个类别方便调用。

UIImageView *imageView;

    __weak __typeof(&*imageView) weakImageView = imageView;
    [imageView sd_setImageWithURL:url
                 placeholderImage:placeholder
                          options:SDWebImageAvoidAutoSetImage // 下载完成后不要自动设置image
                        completed:^(UIImage *image, NSError *error, SDImageCacheType cacheType, NSURL *imageURL) {
        dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0), ^{
            image = [image croppedImage:...]; // 后台线程剪裁图片

            dispatch_async(dispatch_get_main_queue(), ^{
                __typeof(&*weakImageView) strongImageView = weakImageView;
                if (strongImageView) {
                    strongImageView.image = image;
                    [strongImageView setNeedsLayout];
                }
            });
        });
    }];