Here's what I'm using for the MBS site. I'm using it for rollover images, I think it will work in this case though. I'll be happy to look around more if this doesn't work how you need it to.
Put in the <HEAD>
----------------------------------------------------------
<script type=""text/javascript"">
<!--
var preloadFlag = false;
function preloadImages() {
if (document.images) {
// Navigation Buttons
mainNav_over = newImage('images/mainNav_over.gif');
mainNav_down = newImage('images/mainNav_down.gif');
preloadFlag = true;
}
}
-->
</script>
Put in the <BODY>
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<body onLoad="BLOCKED SCRIPTpreloadImages();">
The "BLOCKED SCRIPT" is java_script_: without the "_"