<img   src=http://expert.csdn.net/images/csdn.gif   border=0
  onload= "alert( 'filesize   :   '+   this.fileSize   + '   Byte\r\nwidth   :   '+   this.clientWidth   +   '\r\nheight   :   '+   this.clientHeight) ">

string url = "http://c.csdn.net/bbs/t/5/i/pic_logo.gif";
WebClient clnt = new WebClient();
Byte[] data = clnt.DownloadData(url);
MemoryStream stream = new MemoryStream(data);
Image img = Image.FromStream(stream);
Console.WriteLine("宽：{0}, 高：{1}", img.Width, img.Height);
img.Dispose();
stream.Dispose();
clnt.Dispose();