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