Seria asi?
Código PHP:
<script>
window.onload=function() {var elDiv = document.getElementById('imgContainer');var losFondos = new Array('imagen1.jpg','lafoto.jpg','rutaimagen.jpg','fotonro2.jpg');var indice = Math.floor(Math.random()*losFondos.length);alert(indice);elDiv.style.backgroundImage = 'url("img/'+ losFondos [indice] +'")';}
</script>
<DIV id=box-lateral>
<DIV id=top></DIV>
<DIV id="imgContainer"
style="BACKGROUND-IMAGE: url(http://images.habbos.es/websystem/release_6/box-register/156.gif); BORDER-BOTTOM: #a0a0a0 1px solid; BORDER-LEFT: #a0a0a0 1px solid; PADDING-BOTTOM: 0px; PADDING-LEFT: 5px; WIDTH: 208px; PADDING-RIGHT: 0px; BACKGROUND-POSITION: right bottom; HEIGHT: 175px; BORDER-TOP: #a0a0a0 1px solid; BORDER-RIGHT: #a0a0a0 1px solid; PADDING-TOP: 5px" >
<DIV id=enter><A class=button
href="index.php?s=register"><B><SPAN>Regístrate <IMG
src="http://images.habbos.es/websystem/release_6/flashy_arrow.gif"></SPAN></B><I></I></A>
</DIV></DIV>
<DIV id=bottom></DIV></DIV>
No canvia de imagen..