当点击“上一张”和“下一张”按钮时，图片框里的图片按顺序更换下一张，点一次更换一次。还求一个代码，当点击按钮或图片时随机更换成其它图片。

<style>
#d1{width:200;height:200;border:1 gray solid}
</style>

<body>

<div id=d1></div>
<input type=button id=bn1 value=change>
<span id=s1></span>

<script language="JavaScript">
<!--
a=[
'http://www.biaoqing1.com/uploads/allimg/090508/221T3B08-2.gif',
'http://www.biaoqing1.com/uploads/allimg/090508/221T31E0-14.gif',
'http://biaoqing1.com/uploads/allimg/090508/221T35102-12.gif',
'http://biaoqing1.com/uploads/allimg/090508/221T34R4-0.gif'
]
     var i=0;
     d1.style.background='URL('+a[i]+') no-repeat';

     bn1.onclick=function(){
     ++i>=a.length?i=0:0;
     d1.style.background='URL('+a[i]+') no-repeat';
     s1.innerText=i;
     }
//-->
</script>
</body>