Coś takiego może ?
Kod:
<script langunage="JavaScript">
baner = new Array(3);
baner[0] = '<a href="http://google.pl" target="_blank"><img src="http://google.pl/banner.gf" border="0"></a>';
baner[1] = '<a href="http://google1.pl" target="_blank"><img src="http://google1.pl/banner.gf" border="0"></a>';
baner[2] = '<a href="http://google2.pl" target="_blank"><img src="http://google2.pl/banner.gf" border="0"></a>';
random = Math.floor(Math.random() * baner.length);
document.write(baner[random]);
</script>