Well it might depend on what SEO plugin you are using. Most of them do offer a fallback image if there is no featured image on a post if not it will just pull whatever images are in the post.
It might be best to contact the SEO plugin you are using support forum to know how to set a fallback image. It may be best to just use a site logo for a fallback image.
Im using Yoast. It works fine everywhere else but bbPress.
But, Setting an image fallback per forum category would be a feature that needs to be in bbPress i would assume.
It might be that bbPress does not have support for featured images, and the SEO plugins pull from the featured images.
I think you are the first to mention a feature request for a fallback image in bbPress.
There have been users that were suggesting a possibility for featured images for forums for forum thumbnails/icons. There is a neat little piece of code to that here.
http://www.kristarella.com/2013/04/bbpress-forum-icons/
There might be a way to get the forums featured image to and I guess set it as the child posts (topic) featured image, but right now I am not sure how to do that right now.