I'm just doing this off-the-cuff without testing, so..... *shrug*
Code:
<script type="text/JavaScript">
function rotateBanner()
{
var target = document.images["targetbanner"];
var banners = array("banner1.gif","banner2.gif","banner3.gif");
target.src = Math.floor ( Math.random ( ) * banners.length );
}
</script>
<html>
<body onload="rotateBanner()">
<img src="clearpix.gif" name="targetBanner"/>
</body>
</html>