asp.net mvc控制器动作体返回ImageResult，可作验证码

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;

}