用图片作为label，for属性IE下不起作用

<input type="checkbox" id="a"><label for="a"><img src="..."></label>

window.onload = function(){
if(document.all && navigator.appVersion.indexOf("MSIE")>-1 && navigator.appVersion.indexOf("Windows")>-1)
{
var a = document.getElementsByTagName("label");
for(var i=0,j=a.length;i<j;i++){
if(a[i].hasChildNodes && a[i].childNodes.item(0).tagName == "IMG")
{
a[i].childNodes.item(0).forid = a[i].htmlFor;
a[i].childNodes.item(0).onclick = function(){
var e = document.getElementById(this.forid);
switch(e.type){
case "radio": e.checked|=1;break;
case "checkbox": e.checked=!e.checked;break;
case "text": case "password": case "textarea": e.focus(); break;
}
}
}
}
}
}