use style's background-image property, try
<table border=1>
 <tr>
   <td style="width:160;height:50;" onmouseover="this.style.backgroundImage='url(http://www.csdn.net/images/csdn.gif)';" onmouseout="this.style.backgroundImage='';">csdn</td>
   <td style="width:160;height:50;" onmouseover="this.style.backgroundImage='url(http://msdn.microsoft.com/library/toolbar/3.0/images/banners/msdn_masthead_ltr.gif)'" onmouseout="this.style.backgroundImage='';">msdn</td>
</tr>
</table>