把图片从一个网站下载过来到内存里，然后response出去

先用WebRequest下载图片，再Response出去。
 string imgUrl="http://www.5211314.net/imgs/logo.gif";
 HttpWebRequest request = (HttpWebRequest) WebRequest.Create(imgUrl);
 HttpWebResponse resResult = (HttpWebResponse) request.GetResponse();
 Stream s = resResult.GetResponseStream();

 Bitmap bmp=new Bitmap(s);
 MemoryStream ms = new MemoryStream();
 bmp.Save(ms, ImageFormat.Gif);

 Response.ClearContent();
 Response.ContentType = "image/gif";
 Response.BinaryWrite(ms.ToArray());
 bmp.Dispose();
 Response.End();

如果要判断不同的图片格式：
 string imgType=imgUrl.Substring(imgUrl.LastIndexOf(".")+1).ToLower();
 switch(imgType)
 {
  case "jpg":
   imgType="image/jpeg";
   break;
  case "gif":
   imgType="image/gif";
   break;
  //...........
  default:
   return;
 }