#region 獲取難證碼圖片
        /// <summary>
        /// 獲取難證碼圖片
        /// </summary>
        public void requestVerify()
        {
            try
            {
                HTMLDocument html = (HTMLDocument)this.webLogin.Document.DomDocument;
                IHTMLControlElement img = (IHTMLControlElement)webLogin.Document.Images["verifyImg"].DomElement;

                IHTMLControlRange range = (IHTMLControlRange)((HTMLBody)html.body).createControlRange();
                range.add(img);
                range.execCommand("Copy", false, null);
                img = null;
                range = null;
                html = null;
                if (Clipboard.ContainsImage())
                {
                    this.picVerify.Image = null;
                    this.picVerify.Image = Clipboard.GetImage();

                }
                Clipboard.Clear();
            }
            catch (Exception e)
            {
                throw (new Exception(e.Message + e.StackTrace + e.Source));
                return;
            }
        }
        #endregion

IHTMLControlElement img = (IHTMLControlElement)webLogin.Document.Images["verifyImg"].DomElement;

至于這里找到那個驗證碼圖片的方式很多,可以根所圖片ID,NAME,SRC,TITLE等等...

需引用一個dll ,Microsoft.mshtml