如何在asp.net中直接显示httpwebresponse流中的图像

我想在不保存到文件的情况下直接将从webresponse收到的图像显示到浏览器。我使用了以下代码将图像保存到文件，但我想直接在浏览器中显示。网站在webresponse中提供了我想显示的captcha图像。请帮帮我。

public void captcha(string id, string pass)
{
    HttpWebRequest req;
    HttpWebResponse response;
    string strNewValue,ckuser,ckpass;
    System.Drawing.Image returnval;
    ckuser = id;
    ckpass = pass;
    this.req = (HttpWebRequest)WebRequest.Create("http://site2sms.com/security/captcha.asp");
    ServicePointManager.Expect100Continue = false;
    this.req.CookieContainer = new CookieContainer();
    this.req.AllowAutoRedirect = false;
    this.req.UserAgent = "Mozilla/5.0 (Windows NT 6.1; rv:8.0) Gecko/20100101 Firefox/8.0";
    this.req.Accept = "*/*";
    this.req.Method = "POST";
    this.req.CookieContainer = cookieCntr;
    this.req.ContentType = "application/x-www-form-urlencoded";
    this.req.Referer = "http://site2sms.com/verification.asp?source=login";
    this.strNewValue = "user=" + ckuser + "&password=" + ckpass + "&Submit=Sign+in";
    this.req.ContentLength = this.strNewValue.Length;
    StreamWriter writer = new StreamWriter(this.req.GetRequestStream(), Encoding.ASCII);
    writer.Write(this.strNewValue);
    writer.Close();
    this.response = (HttpWebResponse)this.req.GetResponse();
    returnval = Image.FromStream(response.GetResponseStream());
   // returnval.Save( Server.MapPath("captcha.bmp"));
    Response.AppendHeader("Content-Disposition:", "inline;filename=captcha.bmp");
    Response.ContentType = "image/bmp";
    Response.Write(returnval);
    Response.End();
    this.response.Close();
 }