处理多个翻转器的问题

菜鸟在看<javascript基础教程第七版>
跟着书上的代码写如下，于是在IE8，FF3.0中打开有问题，前面的两个图片（id=flyer,,id=tank）在鼠标停留时没有阴影，
而最后一张（id=helicoptor）可以显示阴影,为什么会这样，，我代码检查好多次没问题，IE也没报错啊
<body bgcolor="#EECC99">

<img src="images/DaVinci.jpg" width="144" height="219" border="0" alt="DaVinci" align="right" hspace="50" />
    <img src="images/leoText.gif" width="375" height="26" border="0" alt="Leonardo's Inventtions" align="left" />
    
    <a href="flyPage.html" class="textField" id="flyer">
    <img src="images/flyer.gif" width="293" height="165" border="0" alt="Flying Machine" vspace="10" id="flyImg" />
    </a><br clear="right" />
    
    <img src="images/bg.gif" width="208" height="27" border="0" id="textField" alt="TextField" align="right" vspace="20" />
    
    <a href="tankPage.html" class="textField" id="tank">
    <img src="images/tank.gif" width="325" height="92" border="0" alt="Tank" id="TankImg" />
    </a><br />
    <a href="helicopter.html" class="textField" id="helicopter">
    <img src="images/helicopter.gif" alt="helicopter" width="224" height="160" border="0" id="helicopterImg" />
    </a> 
</body>
JS代码window.onload = rolloverInit;function rolloverInit(){
for (var i = 0;i<document.links.length;i++){
var linkObj = document.links[i] ;
if(linkObj.className){
var imgObj = document.getElementById(linkObj.className);
if(imgObj){
setupRollover(linkObj,imgObj);
}
}
}
}function setupRollover(thisLink,textImage){
thisLink.imgToChange = new Array;
thisLink.outImage = new Array;
thisLink.overImage = new Array;

thisLink.imgToChange[0] = textImage;
thisLink.onmouseout = rollOut;
thisLink.onmouseover = rollOver;

thisLink.outImage[0] = new Image();
thisLink.outImage[0].src = textImage.src;

thisLink.overImage[0] = new Image();
thisLink.overImage[0].src = "images/"+thisLink.id+"Text.gif";

var rolloverObj= document.getElementById(thisLink.id+"Img");
if(rolloverObj){
thisLink.imgToChange[1] = rolloverObj;

thisLink.outImage[1] = new Image();
thisLink.outImage[1].src = rolloverObj.src;

thisLink.overImage[1] = new Image();
thisLink.overImage[1].src = "images/"+thisLink.id+"_on.gif";
}
}
function rollOver(){
for(var i = 0;i<this.imgToChange.length;i++){
this.imgToChange[i].src = this.overImage[i].src;
}
}function rollOut(){
for(var i = 0;i<this.imgToChange.length;i++){
this.imgToChange[i].src = this.outImage[i].src;
}
}