Animated GIF working on a test server, but not on production server

Hi All,

I have a website where I have a asp:ScriptManager and asp:UptadeProgress:

<asp:UpdateProgressID="UpdateProgress1"runat="server">

<ProgressTemplate>

    <divclass="modal">

    <divclass="center">

    <imgalt=""src="../App_Themes/Default/Images/loader.gif"/>

    </div>

   </div>   

   </ProgressTemplate>

   </asp:UpdateProgress>

During a lengthy operation, the GIF shows correctly in my development environment and a test server, but not on the production server.  The UpdateProgress is blacking out the entire screen and displaying the spinner loader.gif in the middle. On the production server, the blackout does occur, but there is no spinner. Does anyone know what settings on the production server can affect the correct display of the animated GIF?