Show Processing Image On Server side code execution

css
----------------

<style type="text/css">
.HideBackGround
{
background-color: #696969;
filter: alpha(opacity=40);
opacity: 0.7;
xindex:-1;
}
</style>

In Design Page
---------------------------

<asp:UpdateProgress ID="UpdateProgress" runat="server">
<ProgressTemplate>
<asp:Image ID="img_Progress1" ImageUrl="~/NewImages/loading.gif" AlternateText="Processing" runat="server" />
</ProgressTemplate>
</asp:UpdateProgress>
<cc1:ModalPopupExtender ID="modalPopup" runat="server" TargetControlID="UpdateProgress"
PopupControlID="UpdateProgress" BackgroundCssClass="HideBackGround" />
<asp:UpdatePanel ID="pnlData" runat="server" UpdateMode="Conditional" >
<ContentTemplate>

Your table Content

</ContentTemplate>
    </asp:UpdatePanel>

Scripts(to be placed after Updatepanel)
---------------------------------------------------------
    <script type="text/javascript">
        var prm = Sys.WebForms.PageRequestManager.getInstance();
        //Raised before processing of an asynchronous postback starts and the postback request is sent to the server.
        prm.add_beginRequest(BeginRequestHandler);
        // Raised after an asynchronous postback is finished and control has been returned to the browser.
        prm.add_endRequest(EndRequestHandler);
        //     function load() {
        //         Sys.WebForms.PageRequestManager.getInstance().add_endRequest(BeginRequestHandler);
        //         Sys.WebForms.PageRequestManager.getInstance().add_endRequest(EndRequestHandler);
        //     }
        function BeginRequestHandler(sender, args) {
            //Shows the modal popup - the update progress
            var popup = $find('<%= modalPopup.ClientID %>');
            if (popup != null) {
                popup.show();
            }
        }

        function EndRequestHandler(sender, args) {
            //Hide the modal popup - the update progress
            var popup = $find('<%= modalPopup.ClientID %>');
            if (popup != null) {
                popup.hide();
            }
        }
</script>

loading.gif