It looks to me like you want to show a busy animated gif between postbacks, is that right?
Here's how I do it (unless I'm using update panels which have their own progress indicator server control)
<script type="text/javascript">
window.onbeforeunload = function(){showGif();};
</script>
When the page unloads because of a postback (click the button which POSTs to the server), reveal the gif image. When the new page comes back from the server the image will disappear because it's now a new page.
Changing a label is a small operation, so you will probably never see the image as the request/response will turn around very quickly. For testing you can add a System.Threading.Thread.Sleep(3000) in your button handler to simulate a longer running process.
EDIT
AJAX is your only option then, but there are many many ways. One possible solution:
Put the label in an UpdatePanel server control, place button outside of update panel, but set it as an AsyncPostbackTrigger in the update panel's declarative mark-up in the aspx page. Show the gif with OnClientClick handler client-side or using JQuery to attach a client-side click handler to the button.
aspx
<%@ Page Language="vb" AutoEventWireup="false" CodeBehind="Default.aspx.vb" Inherits="AjaxLabel._Default" %>
<!DOCTYPE html>
<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
<title></title>
</head>
<body>
<form id="form1" runat="server">
<asp:ScriptManager runat="server" ID="ScriptManager1"></asp:ScriptManager>
<div>
<asp:UpdatePanel runat="server">
<ContentTemplate>
<asp:Label runat="server" ID="ajaxLabel">Initial value</asp:Label>
</ContentTemplate>
<Triggers>
<asp:AsyncPostBackTrigger ControlID="asyncButton" EventName="Click" />
</Triggers>
</asp:UpdatePanel>
<img id="theImage" src="img/success.png" style="display: none;" />
<asp:Button runat="server" ID="asyncButton" Text="Client & Server Click" OnClientClick="showImage();" OnClick="asyncButton_Click" />
</div>
</form>
<script type="text/javascript">
function showImage() {
document.getElementById('theImage').style.display = "block";
}
</script>
</body>
</html>
aspx.cs
using System;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
namespace AjaxLabel
{
public partial class Default : System.Web.UI.Page
{
protected void Page_Load(object sender, EventArgs e)
{
}
protected void asyncButton_Click(object sender, EventArgs e)
{
ajaxLabel.Text = "Server-side value";
}
}
}
