Try replacing your <body> tag in home.tpl with this:
Code:
{if $main eq "catalog" and $current_category.category eq ""}
<BODY onLoad="javascript:popUp('http://www.mydomin.com/test/test.jpg')">
{else}
<body>
{/if}
Replace the link to the popup with whatever you used. Be sure to make a backup of home.tpl first.