VS 2005 - Disable Form Showing Animated GIF

I have the following code at the bottom of my master page:-

Code:
<div id="overlay">
<asp:Image ID="Image1" runat="server" ImageUrl="~/Images/Disable.gif" style="width: 100%; height:100%; opacity:0.6;filter:alpha(opacity=60)"/>
<div id="IconImage">
<asp:Image ID="Image2" runat="server" ImageUrl="~/Images/loader.gif"/>
</div>
</div>

And here is my CSS:-

Code:
#overlay {
visibility: hidden;
position: absolute;
left: 0px;
top: 0px;
width:100%;

[Code] ....

Here is my javascript:-

Code:
function overlay() {
el = document.getElementById("overlay");
el.style.visibility = (el.style.visibility == "visible") ? "hidden" : "visible";
}

And finally here is my code to trigger the overlay function which is called when I build my data:-

Code:
<asp:ImageButton ID="ImageButton1" runat="server" style="Height:50px; Width: 50px;" ImageUrl="~/Images/Printer.png" ToolTip="Print Report" OnClientClick="overlay()"/>

Everything works fine except my animated gif does not animate. If I load the gif in IE is animates.