One method is to use the forum ID's as the name of your images for each forum. Example:
You have 3 forums: Announcements (forum ID 1), Feedback (forum ID 2) and General Discussions (forum ID 3). Now do the following:
In forumhome_forumbit_level1_post and forumhome_forumbit_level2_post add (generally inside the first <td> tag where the statusicon is located):
Code:
<img src="/path/to/image/$forum[forumid].gif" alt="" border="0" id="forum_statusicon_$forum[forumid]" />
Now go back to your 3 forums. Each is marked by its forum ID. Find the images you want to use. Rename them to the forum ID (e.g., 1.gif, 2.gif, 3.gif), or whichever image extension you use. Now each forum, including the subforums, will have their unique image.
*It's important to set the path of your images correctly with their respective forum IDs to show up correctly.