Captcha Problem

i used captcha in a form.its in update panel. when iam enter wrong code,the msg shows as incorrect code,but the captcha not getting refreshed.

<asp:Image ID="Image1" runat="server" ImageUrl ="random.aspx"/> in random.aspx the captcha code gettting generate.without the usage of updatepanel,the captcha getting refreshed...
whats the problem here??? plz somebody help me. very urgent!!
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