lo probé en los dos browser IExplorer y FireFox y en los dos me anda bien, en ninguno veo esa linita que vos decis...
Código HTML:
<table width="540" style="height: 298" border="0" cellpadding="0" cellspacing="0" id="Marcos">
<tbody>
<tr>
<td width="0" height="0" style="background-image:url(./images/marco_esq_1.gif)"></td>
<td width="540" height="0" style="background-image:url(./images/marco_sup.gif)"></td>
<td width="0" height="0" style="background-image:url(./images/marco_esq_2.gif)"></td>
</tr>
<tr>
<td width="0" height="298" style="background-image:url(./images/marco_izq.gif)"></td>
<td width="540" height="298"></td>
<td width="0" height="298" style="background-image:url(./images/marco_der.gif)"></td>
</tr>
<tr>
<td width="0" height="0" style="background-image:url(./images/marco_esq_3.gif)"></td>
<td width="540" height="0" style="background-image:url(./images/marco_inf.gif)"></td>
<td width="0" height="0" style="background-image:url(./images/marco_esq_4.gif)"></td>
</tr>
</tbody>
</table>
Probalo y fijate.....
