webform中gif转换jpg问题

System.Drawing.Image sourceImage = System.Drawing.Image.FromStream(attachment.SourceStream) ;
//转换为jpg
if (attachment.ContentType != "image/pjpeg")
if (attachment.ContentType != "image/jpeg")
{
sourceImage.Save (ms, System.Drawing.Imaging.ImageFormat.Jpeg) ;....只能转换图片顶端的一部分，其余部分全部是噪点，完全看不清楚