string url = string.Format(@"http://webservice.36wu.com/DimensionalCodeService.asmx/GetCodeImgByString?size={0}&content={1}", 5, 123456);

System.Net.WebRequest webreq = System.Net.WebRequest.Create(url);

System.Net.WebResponse webres = webreq.GetResponse();

using(System.IO.Stream stream = webres.GetResponseStream())

{

   pictureBox1.Image = Image.FromStream(stream);

}