[kh99]

I think you want to edit the forumhome_forumbit_level1_nopost and forumhome_forumbit_level2_nopost templates. Somewhere near the top of each you'll find a {vb:raw forum.title} whcih inserts the title, so you'd have to insert an image tag there. If you arrange things so that the images are named by forumid, then you can probably do it all in the template by using {vb:raw forum.forumid} as part of the file name. Otherwise you'd have to use a plugin to set a variable with the file name.