I’ve heard of people using Facebook for free image hosting, because even if you delete the photo, if you keep the URL where it was located, it will stay there forever. I’m not sure if this is still the case.
It was often said that to properly clean up all your reddit data you should edit your posts before you delete them. Likely the edit will change the record in-place in the DB (as keeping a full edit history would be a nightmare) and deleting the post likely just sets a flag.