I just figured how to preload for those using the secret addon too. I know nuts about scripting but it seems to work hehe
HTML Code:
<script type="text/javascript">
if (document.images)
{
img1 = new Image();
img1.src = "images/<PRELOAD_IMAGE_NAME>";
img1.src = "images/o_<PRELOAD_IMAGE_NAME>";
}
</script>
assuming "o" is the prefix of your secret images (which is the default from that addon, unless you changed it)