在MJPhotoView.m里面添加如下代码：

- (void)scrollViewDidZoom:(UIScrollView *)scrollView {

    CGRect imageViewFrame = _imageView.frame;

    CGRect screenBounds = [UIScreen mainScreen].bounds;

    if (imageViewFrame.size.height > screenBounds.size.height)

    { imageViewFrame.origin.y = 0.0f; }

    else { imageViewFrame.origin.y = (screenBounds.size.height - imageViewFrame.size.height) / 2.0; }

    _imageView.frame = imageViewFrame;

}