关于js切换背景图片的问题

我写的JS方法是：<script language="javascript" type="text/javascript"> function switch(aa){ case 1:document.getElementById("tb").style.background='url(images/op-6.jpg)'; break; case 2:document.getElementById("tb").style.background="url(images/op-5.jpg)"; break; case 3:document.getElementById("tb").style.background="url(images/op-4.jpg)"; break; default:document.getElementById("tb").style.background='url(images/op-6.jpg)'; } </script> 实例化的地方是：<table width=560px height=310px border> <tr><th rowspan=5 id="tb">图片</th> <th onmouseover="switch(1)">1</th></tr> <tr><th onmouseover="switch(2)">2</th></tr> <tr><th onmouseover="switch(3)">3</th></tr> <tr> <th>4</th></tr> <tr> <th>5</th></tr> </table> 请问下高手，我上面的代码问题出在哪？？？？在线等

追问：换成backgroundImage还是不行

其他：把background改成backgroundImage function switch(aa){
      switch (expression){
          case 1:
              ......
         case 2:
              ....


   }
} 你把switch用错了，不要把保留字做函数名
<script language="javascript" type="text/javascript">
function rollpic(aa)
{
switch(aa){
	case 1:
		document.getElementById("tb").style.background='url(images/op-6.jpg)';
		break;
	case 2:
		document.getElementById("tb").style.background="url(images/op-5.jpg)";
		break;
	case 3:
		document.getElementById("tb").style.background="url(images/op-4.jpg)";
		break;
	default:
		document.getElementById("tb").style.background='url(images/op-6.jpg)';
	}
}
</script>
实例化的地方是：
<table  width=560px height=310px border>
<tr><th rowspan=5 id="tb">图片</th>
<th   onmouseover="rollpic(1)">1</th></tr>
<tr><th onmouseover="rollpic(2)">2</th></tr>
<tr><th onmouseover="rollpic(3)">3</th></tr>
<tr>  <th>4</th></tr>
<tr> <th>5</th></tr>
</table>