This is also a simple randomizer...
HTML Code:
<script language="JavaScript">
images = new Array(4);
images[0] = "<a href = 'http://www.LinkToBannersWebsiteA.com' target='_blank'><img src='http://www.PlaceWhereTheBannerIsStored_a.gif' alt='ALT text'></a>";
images[1] = "<a href = 'http://www.LinkToBannersWebsiteB.com' target='_blank'><img src='http://www.PlaceWhereTheBannerIsStored_b.gif' alt='ALT text'></a>";
images[2] = "<a href = 'http://www.LinkToBannersWebsiteC.com' target='_blank'><img src='http://www.PlaceWhereTheBannerIsStored_c.gif' alt='ALT text'></a>";
images[3] = "<a href = 'http://www.LinkToBannersWebsiteD.com' target='_blank'><img src='http://www.PlaceWhereTheBannerIsStored_d.gif' alt='ALT text'></a>";
index = Math.floor(Math.random() * images.length);
document.write(images[index]);
</script>