<script language="JavaScript">
var a=Math.round(Math.random()*2)
path = new Array();
path[0]='<a href="../xxx.php"><body background="pic1.jpg"></a>'
path[1]='<a href="../xxx.php"><body background="pic2.jpg"></a>'  
path[2]='<a href="../xxx.php"><body background="pic3.jpg"></a>'
path[3]='<a href="../xxx.php"><body background="pic4.jpg"></a>'

document.write (path[a]);
</script>