This is not specific to BBpress but you may be able to help out (i do use bbpress!)
Users of my site are hotlinking images in threads, perfectly acceptable behaviour.
However, occasionally the site where the image is coming from is using replacement images – usually porn. The user doesn’t know what has happened because the correct image is stored in their cache.
Is there a way to stop this from happening and check if the image shown corresponds to the image being linked to and if not supply our own replacement?