通过WebRequest上载的Gif图片，为什么是一帧

   HttpWebRequest request = (HttpWebRequest)WebRequest.Create(imgModel.SourceUrl);            request.Referer = Request.UrlReferrer.ToString();            request.UserAgent = Request.UserAgent;            WebResponse response = request.GetResponse();            Stream imgStream = response.GetResponseStream();            if (imgStream != null)            {                Image img = Image.FromStream(imgStream, true, true);                string extension = Common.GetImageFormat(img.RawFormat);                //Bitmap bitmap = new Bitmap(img);//这里用bitmap方式保存也不行                //bitmap.Save(Request.MapPath("/Images/" + Guid.NewGuid().ToString()) + extension, img.RawFormat);                img.Save(Request.MapPath("/Images/" + Guid.NewGuid().ToString()) + extension, img.RawFormat);                WriteAsCrossDomain(Request["jsoncallback"], string.Format("'宽：{0}，高：{1}'", img.Width, img.Height));                img.Dispose();                //bitmap.Dispose();            }