Show Loading Progress Indicator using GIF Image when UpdatePanel is updating in ASP.Net

Show Loading Progress Indicator using GIF Image when UpdatePanel is updating in ASP.Net

HTML Page:
<asp:ScriptManager runat="server">
</asp:ScriptManager>
<asp:UpdateProgress ID="UpdateProgress1" runat="server" AssociatedUpdatePanelID="UpdatePanel1">
<ProgressTemplate>
    <div class="modal">
        <div class="center">
            <img alt="" src="loader.gif" />
        </div>
    </div>
</ProgressTemplate>
</asp:UpdateProgress>
<asp:UpdatePanel ID="UpdatePanel1" runat="server">
<ContentTemplate>
    <div align="center">
        <h1>
            Click the button to see the UpdateProgress!</h1>
        <asp:Button ID="Button1" Text="Submit" runat="server" OnClick="Button1_Click" />
    </div>
</ContentTemplate>
</asp:UpdatePanel>

C# Page:

protected void Button1_Click(object sender, EventArgs e)
{
    System.Threading.Thread.Sleep(5000);
}

CSS:

<style type="text/css">
body
{
    margin: 0;
    padding: 0;
    font-family: Arial;
}
.modal
{
    position: fixed;
    z-index: 999;
    height: 100%;
    width: 100%;
    top: 0;
    background-color: Black;
    filter: alpha(opacity=60);
    opacity: 0.6;
    -moz-opacity: 0.8;
}
.center
{
    z-index: 1000;
    margin: 300px auto;
    padding: 10px;
    width: 130px;
    background-color: White;
    border-radius: 10px;
    filter: alpha(opacity=100);
    opacity: 1;
    -moz-opacity: 1;
}
.center img
{
    height: 128px;
    width: 128px;
}
</style>