if(!IsPostBack)
            {
                ChangeGetCodeImage();
              }



 /// <summary>
        /// 鼠标经过改变获取验证码图片按钮的图片
        /// </summary>
        private void ChangeGetCodeImage()
        {
            this.ibtGetCode.Attributes.Add("onmouseover", "this.src='/Images/getcode2.png'");
            this.ibtGetCode.Attributes.Add("onmouseout", "this.src='/Images/getcode3.png'");
        }