you mean on the front page?
thats in indicator if there is posted so if you change that one you will change every logo as far I know
(its the icon that says:Forum Contains No New PostsForum is LockedRedirect Forum
If you want to change that, you will need to Edit the Sprite for it, you can find these here:
Public_Html/Your Directory/images/forum_icon_sprite.png
Or if you have not set a Custom Directory
Public_Html/images/forum_icon_sprite.png
Bare in mind if you edit these, it will edit the Entire forum, and not just the Front Page.
Hope this helps!