<script language="javascript" type="text/javascript">
    var img = new Array("http://www.site.it/img1.jpg", "http://www.site.it/img2.jpg", "http://www.site.it/img3.jpg");
    var x = Math.floor(img.length * Math.random(1));
    document.write('<img border="0" class="banner" src=\"'+ img[x] +'\" </img>');
</script>