If this ever comes near reality, I would strongly oppose to store the images on-chain or in the main torrent. That could attract the wrong people. Images can still be included via external servers like talkimg, or via external image torrents.
In that case the size of torrent can be pretty small and anyone can easily seed it and others can download it and become its seeders. I believe the one who may create such torrent should take responsibility to create more torrent based backups on weekly or monthly basis so each new post/thread of the forum can be updated and the downloaders can get updated data easily.
It would be even better if the images can be uploaded on external servers because that way the downloads can easily browse the forum's posts/threads and may click on that external links. TalkImg as we all know is a great option for the purpose and I believe Joker_josue will allow such type of uploading of images on his site.