如何更换图片？？

<table width="100" border="1" cellspacing="0" cellpadding="0">
  <tr onclick="mm(this)">
    <td><img src="images/arr2.gif" width="5" height="9"></td>
    <td>&nbsp;</td>
    <td>&nbsp;</td>
    <td>&nbsp;</td>
  </tr>
  <tr onclick="mm(this)">
    <td>&nbsp;</td>
    <td>&nbsp;</td>
    <td>&nbsp;</td>
    <td>&nbsp;</td>
  </tr>
  <tr onclick="mm(this)">
    <td>&nbsp;</td>
    <td>&nbsp;</td>
    <td>&nbsp;</td>
    <td>&nbsp;</td>
  </tr>
</table><SCRIPT LANGUAGE="JavaScript">
<!--
var global;
function mm(tr)
{
  if(global && global.tagName=="TR") global.cells[0].innerHTML = "&nbsp;";
  global = tr;
  tr.cells[0].innerHTML = "<img src='images/arr2.gif' width='5' height='9'>";
}
//-->
</SCRIPT>