[austriaka+]

You are on the right track, but you don't need the <PRELOAD_IMAGE_NAME>.
Just use

<script type="text/javascript">
if (document.images) 
{
   img1 = new Image();
   img1.src = "images/o_<IMAGE_NAME>";
}
</script>

You don't need the Image Pre-Loader Addon since you are only interested in the actual images name (<IMAGE_NAME>)