System.Drawing.Image.FromStream(webStream) parameter is not valid in sharepoint

Hallo,

I want todownloadapicture from SharePoint, Itested thecode inconsole and that's good butatwebpartI habe alwayserrorat

image =System.Drawing.Image.FromStream(webstream)Parameteris not valid

 HttpWebRequest httpWebRequest = (System.Net.HttpWebRequest)System.Net.HttpWebRequest.Create(imageUrl);
                httpWebRequest.UseDefaultCredentials = true;
                httpWebRequest.PreAuthenticate = true;
                httpWebRequest.Credentials = CredentialCache.DefaultCredentials;
                httpWebRequest.AllowWriteStreamBuffering = true;
                httpWebRequest.UserAgent = "Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 5.1)";
                httpWebRequest.Referer = "http://www.google.com/";
                System.Net.WebResponse webResponse = httpWebRequest.GetResponse();
                System.IO.Stream webStream = webResponse.GetResponseStream();

                image = System.Drawing.Image.FromStream(webStream);
                webResponse.Close();
                webResponse.Close();