请帮忙修改成按顺序循环变换变景Javascript程序

三张JPG图片:A1.JPG,A2.JPG,A3.JPG 
下边是一个，按一定时间内随机变换table表格图片背景脚本<html>
<head>
<style>
.pic0{background-image:url(img/A1.jpg);}
.pic1{background-image:url(img/A2.jpg);}
.pic2{background-image:url(img/A3.jpg);}
</style>
</head>
<body>
<script language="Javascript">
function change_img()
{
t.className='pic'+(Math.floor(Math.random()*3));
}
setInterval("change_img()",1000);
</script>
<table width="1058" height="491" border="0" cellpadding="0" cellspacing="0" background="img/A1.jpg" id="t">
    <tr>
      <td align="center">
        &nbsp;</td>
        </tr>
      </table>
</body>
</html>请帮忙修改成按顺序1,2,3...循环变换变景Javascript程序