prova così se ho intuito il problema:
codice:
<html>
<head>
<script>
var max=3; //numero di img
function cambia(){
c=document.getElementById("banner").style.backgroundImage;
d=parseInt(c.charAt(4));
d=(d<max) ? (d+1) : (d=1);
c="url("+d+".jpg)";
document.getElementById("banner").style.backgroundImage=c;
setTimeout('cambia()',1500);
}
</script>
</head>
<body onLoad="setTimeout('cambia()',1500)">
<table border="1">
<tr>
<td id="banner" width="100" height="400" style="background-image:url(1.jpg)"></td>
</tr>
</table>
</body>
</html>
Le X img le devi numerare da 1 a max