This hack has worked well for some time now at ENWorld, though we use it in the $header file to construct a forum specific logo. Our header looks like this...
Code:
<if condition="$foruminfo['banner'] AND $foruminfo['bannerlink']">
<a href="$foruminfo[bannerlink]" target="_blank">
<img src="images/banner/$foruminfo[banner]" alt="$foruminfo[banner]" border="0" />
</a><else />
<if condition="$foruminfo['banner']">
<img src="images/banner/$foruminfo[banner]" alt="$foruminfo[banner]" border="0" />
<else />
<img src="images/banner/default.gif" border="0">
</if></if>