I was wondering whether the "3-image" limit applies to a single post or across all posts. I haven't hit the limit yet in either case but if it's the latter it might become an issue.

It's 3 in a single post 🙂 . I think (if I remember) there is no limit with how many posts with the 3 attachments you can make. We don't restrict how many attachments a member can have overall.

There is a max file size as well per each upload, I have never hit it but there is a fairly high mb limit.

The subscribers help fund the storage of the forum, so there are some limits to keep the space used manageable. Subscribers can have more attachments per post.