Control Panel --> General --> HTML Includes --> Default header
put in something like:
<div class="header_logo">
<a href="http://yoursite.com/ubbthreads/ubbthreads.php?ubb=cfrm">
<img src="http://yoursite.com/ubbthreads/logos/yourlogo.png" />
</a>
</div>
puts a clickable logo in your header..
you can style the .header_logo class in the extra CSS to make it look however you want (optional)