以下介绍两种方法：
1.利用WebRequest，WebResponse 类
WebRequest wreq=WebRequest.Create("http://www.xueit.com/eimg/uploadfile/downpig/20098/098215331763.gif");
    HttpWebResponse wresp=(HttpWebResponse)wreq.GetResponse();
    Stream s=wresp.GetResponseStream();    
    System.Drawing.Image img;
    img = System.Drawing.Image.FromStream(s);  
    img.Save("D:\\aa.gif",ImageFormat.Gif);   //保存
//下面直接输出
MemoryStream ms=new MemoryStream();
    img.Save(ms,ImageFormat.Gif);
    img.Dispose();
    Response.ClearContent();
    Response.ContentType="image/gif";
    Response.BinaryWrite(ms.ToArray());

2.利用 WebClient 类
WebClient my=new WebClient();
    byte[] mybyte;  
    mybyte=my.DownloadData("http://www.xueit.com/eimg/uploadfile/downpig/20098/098215331763.gif");            MemoryStream ms=new MemoryStream(mybyte);   
    System.Drawing.Image img;
    img = System.Drawing.Image.FromStream(ms);  
    img.Save("D:\\a.gif",ImageFormat.Gif);   //保存
//下面直接输出
Response.ClearContent();
    Response.ContentType="image/gif";
    Response.BinaryWrite(mybyte);
如果是真实的图片地址直接用
my.DownloadFile("http://www.xueit.com/eimg/uploadfile/downpig/20098/098215331763.gif","D:\\a.gif");   //保存
直接可以保存