please wait image on every post back in asp.net using ajax

//css

.modalPopup
{
background-color: #696969;
filter: alpha(opacity=40);
opacity: 0.7;
xindex:-1;
}

//javascript
<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 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>

//ajax
<asp:UpdateProgress ID="UpdateProgress" runat="server">
<ProgressTemplate>
<asp:Image ID="Image1" ImageUrl="images/wait.gif" AlternateText="Processing" runat="server" />
</ProgressTemplate>
</asp:UpdateProgress>
<asp:ModalPopupExtender ID="modalPopup" runat="server" TargetControlID="UpdateProgress"
PopupControlID="UpdateProgress" BackgroundCssClass="modalPopup" />