<script language="javascript">
function cambiarBG(celda, imagen) {
celda.style.backgroundImage = "url("+imagen+")";
}
</script>
<table>
<tr>
<td onMouseOver="cambiarBG(this, 'imagen_over.gif');" style="background-image:url('logo_i.gif');">
</td>
</tr>
</table>