I think this happens at times when the photos are posted to a site elsewhere, then linked to a post here. When the site elsewhere falls into disuse, the account is closed and a dead link results. Yep, it's a letdown to find a thread that fulfills a quest and the missing pictures make it useless.
This is as much a user issue as a board issue. When making new posts, don't use a remote hosting site for your pics. When doing my posts, I scale my pics to 800p wide, and upload them with my post, so the images are on the forum site, not remotely located. Pics will never get dead links that way.