Image in Nested Update panel is not getting refreshed on postback

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VB

ASP.NET

Hi..!

I have two nested updatepanels in my aspx page.

In the child update panel, i have an image control.
I'm using an image button to refresh the image of the image control in the code behind.

But, its not working as i'm expecting. Here is my .aspx page code,

<asp:UpdatePanel ID = "up1" runat = "server" >
   <ContentTemplate>
   <asp:UpdatePanel ID = "xxx" runat = "server" >
   <ContentTemplate>
      <fieldset style="padding:10px;width:30%;">

         <asp:Image ID="ImageCaptcha" ImageUrl="~/captcha.ashx"  
            runat="server" Height="50px" AlternateText="If you can't 
            read this number refresh your screen" Width="124px" /><br />

         <asp:ImageButton ID="btnRefresh" runat="server" 
            Width="20px" TabIndex="17" 
            ImageUrl="~/Images/refreshimg.jpg" Height="16px" 
            ToolTip="Refresh Code" meta:resourcekey="btnRefreshResource1" 
            CausesValidation="true" />
        
         <asp:Button ID="btnSubmit" runat="server" 
            Height="28px" meta:resourcekey="btnSubmitResource1" 
            Text="Create" Width="109px" TabIndex="18" />

      </fieldset>
   </ContentTemplate>
</asp:UpdatePanel>
<pre>&lt;/ContentTemplate&gt;
&lt;/asp:UpdatePanel&gt;

Code Behind is,

Protected Sub btnRefresh_Click(ByVal sender As Object, ByVal e As System.Web.UI.ImageClickEventArgs) Handles btnRefresh.Click

       ImageCaptcha.ImageUrl = "~/logo.jpg"
   End Sub