How to add this image on index:
You can add an image to the forumbit_depth1_forum_lastpost and forumbit_depth2_forum_lastpost templates.
I added this code in forumbit_depth2_forum_lastpost
Why I need to enter this code in forumbit_depth1_forum_lastpost?
Maybe you don't need that other one then, if it's showing where you need it after adding it to forumbit_depth2_forum_lastpost, should be enough.