Today, I learned that the coconut crab is the largest land-living arhropod in the world, Clinophobia is a fear of going to bed, and you can easily cause an animated gif to display while your ASP.NET AJAX Update panel is busy doing a call back.
It's easy, just use the UpdateProgress Control. Point it at the update panel and give it something to display and voila! Here's a code example:
<asp:UpdateProgress ID="updateProgress1" runat="server" AssociatedUpdatePanelID="UpdatePanel1" DisplayAfter="100" DynamicLayout="true">
<div class="Update"><img src="MyAnimated.gif" /></div>