JS Invisible URL Hit Cycle SRC

<html>
<head>
<script>
var opainc=0.2;
var counterrep=0;
var banners=['https://s0.2mdn.net/viewad/2070351/728x90_MobileDev.jpg','https://s0.2mdn.net/viewad/2070351/A+Mind+For+Code_728x90.jpg'];
var counter=0;
function run(){
setInterval(cycle,2000);
}
function cycle(){
counter++;
if(counter==banners.length) {
counter=0;

}
document.getElementById("img").src=banners[counter];
counterrep++;
document.body.innerHTML=(document.body.innerHTML)+'<br>URL #:'+counterrep+'<br>';
}
</script>
<body onload="script:run();">
<iframe id="img" style="width:0;height:0;opacity:0.0;">
</body>
</html>