.header {
position: absolute;
left: 50%;
top: 0px;
width: 800px;
z-index: 1;
margin-left: -400px;
}

<div class="header">

<table cellpadding="0" cellspacing="0" width="800" align="center">
 <tr>
  <td><img src="http://p4.atlasservers.com/~jmyspace/adbg.jpg" />
  </td>
 </tr>
 </table>

</div>