Here's a simple HTML solution; adjust accordingly.
Add to Control Panel » Display Options » HTML Includes » Default Header:

Code
<br />
<table align="center" width="800">
<tr>
<td align="center" valign="middle" width="325">
<a href="http://YOUR_FORUM_URL/" target="_top">
<img alt="FORUM NAME" width="300" height="50" 
     src="YOUR_FORUM_LOGO" border="0"></a>
</td>
<td align="center" valign="middle">

<!--// This gives you another column for ads, etc //-->

</td>
</tr>
</table>
<br />

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