Dit zet je in je HEAD:
<script language="javascript" type="text/javascript"> 
<!-- Script afkomstig van Sitemasters.be
  link = new Array() 
  bann = new Array() 
    //Pas hier alles aan, je kan nog extra links en banners toevoegen.
    link[0] = "adverteren.php"; 
    bann[0] = "banner.gif"; 
        
    link[1] = "adverteren.php"; 
    bann[1] = "banner2.gif"; 
	
    link[2] = "http://www.2link.be"; 
    bann[2] = "2link.gif"; 
	
    link[3] = "http://www.lach.nu"; 
    bann[3] = "lachnu.gif"; 

  var i = Math.floor(Math.random() * bann.length); 

function showbanner() { 
  if(i < bann.length-1){ 
    i ++;
  } 
  else{ 
    i = 0
  } 
  var samen = "<a href='" + link[i] + "' target='_blank'>" + 
              "<img src='" + bann[i] + "' width='468' height='60' border='0'></a>"; 
         
  if(document.getElementById) { 
    document.getElementById('banner').innerHTML = samen; 
  } 
         
  else if(document.all) { 
    document.all[banner].innerHTML=samen; 
  } 
  
  setTimeout('showbanner()',15000); 
 } 
	
window.onload = showbanner ();
// einde script -->
</script> 


Het volgende zet je in je BODY:
<div id="banner"> 
<script language="javascript" type="text/javascript"> 

document.write("<a href='" + link[i] + "' target='_blank'>" + 
               "<img src='" + bann[i] + "' width='468' height='60' border='0'></a>"); 

</script>
</div>