怎么会提示参数无效

<html>
<head>
<title>img</title>
<script type="text/javascript">
window.onload=function () {
var oImg=document.getElementsByTagName("img");
oImg[1].style.display="none";
}
function handle(oEvent) {
var oImg=document.getElementsByTagName("img");
if (oEvent.type=="mouseover")
{
oImg[0].style.display="none";
oImg[1].style.display="block";
oImg[1].style.left=oEvent.clientX+5;
oImg[1].style.top=oEvent.clienty+5;
}
else if(oEvent.type=="mouseout")
{
oImg[1].style.display="none";
return oImg[0].style.display="inline";
}
}
</script>
</head><body>
<img onmouseover="handle(event)" src="http://t2.baidu.com/it/u=3117861305,1790012307&fm=0&gp=2.jpg">
<img onmouseout="handle(event)" src="http://img.ganxiu.com/data/i2007/7f7e0e17cf69fdb2.jpg">
</body>
</html>