thats a pretty easy script to make yourself if you hardcode the images and address
anyway, heres a javascript for you
<SCRIPT LANGUAGE="JavaScript">
function random_banner(){
var i=0;
banners = new Array();
banners[0] = '<IMG SRC="banner0.gif" WIDTH=400 HEIGHT=40>';
banners[1] = '<IMG SRC="banner1.gif" WIDTH=400 HEIGHT=40>';
banners[2] = '<IMG SRC="banner2.gif" WIDTH=400 HEIGHT=40>';
today = new Date();
i=today.getSeconds();
n=banners.length;
return banners[i-Math.round((i-1)/n)*n];
}
</SCRIPT>
<SCRIPT LANGUAGE="JavaScript">
document.write(random_banner())
</SCRIPT>
in the array above just add the banners you want