初学者问个JS图片处理的问题

Question

把你的东西改的面目全非了 哈哈 这中效果建议直接用绿镜，简单有高效。
如果测试有bug 麻烦给我说一下。。
<script type="text/javascript"> 
var timer=""
var a=0  //判断是不是第一次按按纽
var toumingdu =0
var cun=""
var imga = "http://img1.qq.com/news/pics/13709/13709646.jpg"; 
var imgb = "http://img1.qq.com/news/pics/13710/13710242.jpg"; 
var imgc = "http://img1.qq.com/news/pics/13702/13702170.jpg"; 
var img = [imga,imgb,imgc]
function fade(e) 
{
if(document.getElementById("img").src==img[e])
return
clearTimeout(timer)
if(a==0)
{toumingdu =0}
else
{toumingdu =100}
if(a==0)
  {document.getElementById("img").src= img[e]
  a=1
   show()
  }
  else
  {
  cun=img[e]
  xiaoshi()
  }
}
function show(){
if(toumingdu==100)
{clearTimeout(timer)}
else
{toumingdu=toumingdu+10
document.getElementById("img").style.filter="alpha(opacity="+toumingdu+")"
timer=setTimeout("show()",100)}
}function xiaoshi(){
if(toumingdu==0)
{clearTimeout(timer)
document.getElementById("img").src= cun
show()
}
else
{
toumingdu=toumingdu-10
document.getElementById("img").style.filter="alpha(opacity="+toumingdu+")"
timer=setTimeout("xiaoshi()",100)}
}
</script> 
<img id="img" src="a" style="filter:alpha(opacity=0); border:0; width:310px; height:210px"  style="cursor:pointer"  /> 
<img src="http://www.91wang.com/images/submenu_isell.gif" name="a1" onclick="fade(0)" style="cursor:pointer" /> 
 <img src="http://www.91wang.com/images/submenu_isell.gif" name="b1" onclick="fade(1)" style="cursor:pointer" />
 <img src="http://www.91wang.com/images/submenu_isell.gif" name="c1" onclick="fade(2)" style="cursor:pointer" />