Modal backgroud on page loading progress is not working

In aspx page I have dynamically loaded user control(ascx). Everything is working correctly.But it takes too much time due to large amount of time.
Now I try to add Page loading progress in aspx page, its not working.
I use a asp:panel to show modal backgroud when loading.
In aspx page:
<asp:Panel ID="Panel1" runat="server" Height="900px" Width="100%" CssClass="divModalBackground2" Visible="true" >
        <asp:Image runat="Server" ID="ImageLoader" CssClass="LoadingProgress" ImageUrl="../../App_Themes/Black/images/ajax-loader.gif" />
 </asp:Panel>
In my cs page I put the code like this:
//to show modal popup

protected override void OnInit(EventArgs e)
    {
        base.OnInit(e);
     ...