Jak odczytać adres obrazka reCAPTCHA?

public void WriteNewTopic(string subject, string message)
        {
            // GET - pobranie strony formularza, w celu wyciągnięcia tokenu zabezpieczającego
            _webRequest = (HttpWebRequest) WebRequest.Create(this.Url + "newthread.php?do=newthread&f=" + AppsId);
            _webRequest.Method = "GET";
            _webRequest.Headers["Cookie"] = this.Cookie;
            _webRequest.UserAgent = "Mozilla/5.0 (Windows NT 5.1; rv:2.0) Gecko/20100101 Firefox/4.0";
            _webRequest.ContentType = "application/x-www-form-urlencoded";
            _webRequest.KeepAlive = true;
            _webRequest.Accept = "text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8";
            _webRequest.Referer = this.Url + "viewforum.php?f=" + this.AppsId;
            _webRequest.AllowAutoRedirect = true;

            // Odpowiedź serwera
            HttpWebResponse response = null;
            try
            {
                response = (HttpWebResponse) _webRequest.GetResponse();
            }
            catch (WebException webException)
            {
                MessageBox.Show("Wystąpił błąd serwera: " + webException.Response);
                return;
            }
            StreamReader reader = new StreamReader(response.GetResponseStream());
            string sourceCode = reader.ReadToEnd();

            // Wyciąganie tokenu
            Match match = Regex.Match(sourceCode, "var SECURITYTOKEN = \"(.*)\""); // var SECURITYTOKEN = "1301771420-354a766e96ce7e3922e2e73eaa3ebc2bca5b4ff9";
            string securityToken = match.Groups[1].ToString();

            

            Regex rgx = new Regex("challenge.k=(.[^ ]{40})");
            Match matchCaptchaJavaScript = rgx.Match(sourceCode);
            string matchCaptchaJavaScriptUrl = matchCaptchaJavaScript.Groups[1].ToString();

            string noscriptUrl = "http://www.google.com/recaptcha/api/challenge?k=" + matchCaptchaJavaScriptUrl;

            // Wyciągnięcie adresu obrazka

            _webRequest = (HttpWebRequest)WebRequest.Create(noscriptUrl);
            _webRequest.Method = "GET";
            _webRequest.Headers["Cookie"] = this.Cookie;
            _webRequest.UserAgent = "Mozilla/5.0 (Windows NT 5.1; rv:2.0) Gecko/20100101 Firefox/4.0";
            _webRequest.ContentType = "application/x-www-form-urlencoded";
            _webRequest.KeepAlive = true;
            _webRequest.Accept = "text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8";
            _webRequest.Referer = this.Url + "viewforum.php?f=" + this.AppsId;
            _webRequest.AllowAutoRedirect = true;

            // Odpowiedź serwera
            HttpWebResponse response2 = null;
            try
            {
                response2 = (HttpWebResponse)_webRequest.GetResponse();
            }
            catch (WebException webException)
            {
                MessageBox.Show("Wystąpił problem z odczytem reCaptcha: " + webException.Response);
            }
            StreamReader reader2 = new StreamReader(response2.GetResponseStream());
            string sourceCode2 = reader2.ReadToEnd();

            // challenge : '03AHJ_VutwWaXxLQ3Aj0fWYorSZ8XvskGWCSef5vB5MHFVb8_HpIZgXt1nabbzMV582mVjHpiU3Yafm3PM0QPO2SFMdXFGzr0mKg2p-bJxE3s7DxNAyPXJK3kkIjSeLZZrfs4SWc68P2Q4XS8taXbFgDr8Gq_73diEKg'
            Match recaptchaChallengeField = Regex.Match(sourceCode2, "challenge : '(.+)'");
            string recaptchaChallenge = recaptchaChallengeField.Groups[1].ToString();

            string recaptchaImageAddress = "http://www.google.com/recaptcha/api/image?c=" + recaptchaChallenge;

            WebClient webClient = new WebClient();
            webClient.DownloadFile(recaptchaImageAddress, "lastRecaptcha.jpg");


            reCaptchaReader reCaptchaReaderForm = new reCaptchaReader();
            reCaptchaReaderForm.ShowDialog();
            string recaptchaImageCode = reCaptchaReaderForm.Code;
            // ------------------------------------------------------------

            // POST - wysłanie formularza
            _webRequest = (HttpWebRequest) HttpWebRequest.Create(this.Url + "newthread.php?do=newthread&f=" + AppsId);
            _webRequest.Method = "POST";
            _webRequest.Headers["Cookie"] = this.Cookie;
            _webRequest.UserAgent = "Mozilla/5.0 (Windows NT 5.1; rv:2.0) Gecko/20100101 Firefox/4.0";
            _webRequest.ContentType = "application/x-www-form-urlencoded";
            _webRequest.KeepAlive = true;
            _webRequest.Accept = "text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8";
            _webRequest.Referer = this.Url + "viewforum.php?f=" + this.AppsId;

            string postData =
                "do=postthread&f=" + this.AppsId +
                "&wysiwyg=0&subject=" + subject +
                "&message=" + message +
                "&securitytoken=" + securityToken +
                "&recaptcha_challenge_field=" + recaptchaChallenge +
                "&recaptcha_response_field=" + recaptchaImageCode;

            byte[] buffer = Encoding.UTF8.GetBytes(postData);
            _webRequest.ContentLength = buffer.Length;
            Stream dataStream = _webRequest.GetRequestStream();
            dataStream.Write(buffer, 0, buffer.Length);
            dataStream.Close();

            try
            {
                _webRequest.GetResponse();
            }
            catch (WebException webException)
            {
                MessageBox.Show("Wystąpił błąd serwera: " + webException.Response);
            }
        }