Forse ho risolto cos':

codice:
<html>
<head>
<script type="text/javascript">
<!--
var colori = new Array("url(1.gif)","url(2.gif)","url(3.gif)");
var n = 0;
function cambiaColore() {
n++;
if (n>=colori.length) n = 0;

 	document.body.style.backgroundImage = colori[n];
}
-->
</script>
</head>
<body bgcolor="#ffffff" onload="setInterval('cambiaColore()',1000);">