I use a javascript on my page to accomplish this:
Put this in the HEAD of your site...
<script language="JavaScript" type="text/javascript">
<!-- Hide from old browsers
iArray=new Array(
"images/Pic1.jpg",
"images/Pic2.jpg",
"images/Pic3.jpg",
"images/Pic4.jpg",
"images/Pic5.jpg");
ri=Math.floor(iArray.length*Math.random());
ri='<img src="'+ iArray[ri]+ '" border=0>';
// end hiding -->
</script>
Put this where you want the images to display...
<script language="JavaScript" type="text/javascript">
<!--
document.write(ri);
// -->
</script>
Hope this is helps
DBigger