public ActionResult Img()
{
// 获取博客园空间顶部的banner图片
WebRequest req = WebRequest.Create("http://space.cnblogs.com/images/a4/banner.jpg");
WebResponse res = req.GetResponse();
Stream resStream = res.GetResponseStream();
Image img = Image.FromStream(resStream);

// 输出给客户端
ImageResult r = new ImageResult(img);
return r;
}

asp.net mvc控制器动作体返回ImageResult，可作验证码