新手 JS 特效 小问题!

<html> 
<head> 
<title>下拉选项游戏图片切换 </title> 
<script language="javascript"> 
function showPhoto(k) {
var obj=document.getElementById("objdiv").getElementsByTagName("img"),j=obj.length;
for (var i=0;i<j;i++)
obj[i].style.display="none"
if (k>0)obj[k-1].style.display="block"
} 
</script>
<style>
#objdiv img{display:none}
</style> 
</head> 
<body> 
<form name="myform"> 
<select name="game" onchange="showPhoto(this.selectedIndex)"> 
<option selected="selected">--请选择一个游戏-- </option> 
<option>大海战2 </option> 
<option>大话西游 </option> 
<option>热血江湖 </option> 
<option>大航海时代 </option> 
</select> 
<div id="objdiv" align="center"> 
<img src=http://tucoo.com/icon/Yazoo%20smilies/s/Love.png> 
<img src=http://tucoo.com/icon/Yazoo%20smilies/s/Cool.png> 
<img src=http://tucoo.com/icon/Yazoo%20smilies/s/Crazy.png> 
<img src=http://tucoo.com/icon/Yazoo%20smilies/s/Embarrassed.png> 
</div> 
</form> 
</body> 
</html>