You may put the following script for preloading the menu images, to your html code before the </head> tag:

<script language=javascript>
var preloadimages=new Array("image1.gif","image2gif");
var theimages=new Array();

for (p=0;p<preloadimages.length;p++){
theimages[p]=new Image();
theimages[p].src="/awmData-menu/"+preloadimages[p];}
</script>