<?if($AVATAR_URL$)?>  
<IMG src="$AVATAR_URL$" border="0" />  
<?else?>  
<script type="text/javascript">  
ava=Math.floor(Math.random() *10);  
document.write('<img src="http://путь_к_папке_с_аватарами/ava'+ava+'.gif" border="0" />');  
</script>  
<?endif?>