How to reload the captcha.aspx

Hi everyone this is my first thread is this forum

I have some question about refreshing the captcha.

I created a captcha.aspx then I call it on the image control

ImgCaptcha.ImageUrl = "~/CaptchaControl.aspx";

to validate i use session syntax

 if (Session["captchastring"].ToString().Equals(TxtCaptcha.Text.ToString()))

now this is the challenge on how to refreshing the captcha

I've been put it already to the updatepanel but still not functioning rather than if I reload the whole pages

here what I've done so far: