[Opserty]

Something like:

<if condition="$foruminfo['forumid']">
    <img alt="your site address" src="$stylevar[imgdir_misc]/header_f$foruminfo[forumid].png" width="550" height="100" border="0" /> 
<else />
    <!-- DISPLAY YOUR DEFAULT HEADER HERE -->
</if>

Would have it so that it would be displayed if user is browsing a particular forum.

<if condition="in_array($foruminfo['forumid'], array( 5, 6, 4, 9, 11))">
    <img alt="your site address" src="$stylevar[imgdir_misc]/header_f$foruminfo[forumid].png" width="550" height="100" border="0" /> 
<else />
    <!-- DISPLAY YOUR DEFAULT HEADER HERE -->
</if>

Allows you to set which forums you would like to use the custom header for. (Change the forumids in Red and separate them with commas.)