Confusion about UpdatePanel Loading image

I am trying to learn how to use UpdatePanel And this is my code.The problem is everything is ok but I cant see Loading.gif image  .when code excatues Its wisible  aspx page  .What am i missing out here                    <div>         <asp:ScriptManager ID="ScriptManager1" runat="server"></asp:ScriptManager>         <asp:UpdatePanel ID="UpdatePanel1" runat="server">             <ContentTemplate>                 <asp:Button ID="Button1" runat="server" Text="Button" OnClick="Button1_Click" />         ...