Ok, I tried some stuff as well and here's what I've discovered thus far.
1. Putting the new <tr> in "forumhome_forumbit_level2_post" places an image after each
forum, not after each
category.
2. In "forumhome_forumbit_level2_nopost" I can't even find where that code is used, because I see no results.
3. In "forumhome_forumbit_level1_post" I can't see where it has any effect at all (same thing as #2).
4. In "forumhome_forumbit_level1_nopost" the image is added right after the forum title bar (aka, "forum," "posts," "thread," etc.
I'm still of the opinion that this may have to be done with a hack.