菜鸟请教(为什么第二次就不能翻转图片了呢?)

为什么第二次就不能翻转图片了呢?<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
 <head>
  <title> new document </title>
  <meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
  <meta name="author" content="" />
  <meta name="keywords" content="" />
  <meta name="description" content="" />
  <script type="text/javascript" src="train.js"></script> </head> <body>
    <div>
<img id="adBanner" src="01.jpg" width="307" height="203" alt="" />
    </div>
 </body>
</html>
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//jsvar adImages = new Array("01.jpg","02.jpg","03.jpg");
var thisAd = 0;
 function rotate(){
 document.getElementById("adBanner").src = adImages[thisAd];
    thisAd++;
    setTimeout("rotate()", 3000);
 }
window.onload = rotate;