It looks as if the bars are the same as your table headers so how about this...
HTML Code:
<table class="tborder" cellpadding="$stylevar[cellpadding]" cellspacing="$stylevar[cellspacing]" border="0" width="100%" align="center">
<tr>
<td class="thead" width="100%"></td>
</tr>
</table>
<table align="center">
<tr>
<td>
<img src="/image directory/your ad image.gif" /></td>
</tr>
</table>
<table class="tborder" cellpadding="$stylevar[cellpadding]" cellspacing="$stylevar[cellspacing]" border="0" width="100%" align="center">
<tr>
<td class="thead" width="100%"></td>
</tr>
</table>