//调用示例
downRemoteImg("Images", "http://common.cnblogs.com/images/qdigg.gif");

public string downRemoteImg(string savedir, string imgpath)
        {
            if (string.IsNullOrEmpty(imgpath))
            {
                return string.Empty;
            }
            else
            {
                string imgName = string.Empty;
                string imgExt = string.Empty;
                string saveFilePath = string.Empty;
                imgName = imgpath.Substring(imgpath.LastIndexOf("/"), imgpath.Length - imgpath.LastIndexOf("/"));
                imgExt = imgpath.Substring(imgpath.LastIndexOf("."), imgpath.Length - imgpath.LastIndexOf("."));

                saveFilePath = Server.MapPath(savedir);
                if (!Directory.Exists(saveFilePath))
                    Directory.CreateDirectory(saveFilePath);
                try
                {
                    WebRequest wreq = WebRequest.Create(imgpath);
                    wreq.Timeout = 10000;
                    HttpWebResponse wresp = (HttpWebResponse)wreq.GetResponse();
                    Stream s = wresp.GetResponseStream();
                    System.Drawing.Image img;
                    img = System.Drawing.Image.FromStream(s);
                    switch (imgExt.ToLower())
                    {
                        case ".gif":
                            img.Save(saveFilePath + imgName, ImageFormat.Gif);
                            break;
                        case ".jpg":
                        case ".jpeg":
                            img.Save(saveFilePath + imgName, ImageFormat.Jpeg);
                            break;
                        case ".png":
                            img.Save(saveFilePath + imgName, ImageFormat.Png);
                            break;
                        case ".icon":
                            img.Save(saveFilePath + imgName, ImageFormat.Icon);
                            break;
                        case ".bmp":
                            img.Save(saveFilePath + imgName, ImageFormat.Bmp);
                            break;
                    }
                    img.Dispose();
                    s.Dispose();
                    return savedir + imgName;
                }
                catch
                {
                    return imgpath;
                }
            }
        }