On Mastodon, like kbin and lemmy, locally posted images are hosted locally, and remotely hosted images are cached locally. This can cause a lot of storage. But you can run an automated cron that will clear out cached images older than whatever you want, like a month. Then the local cached storage is at a minimum, and if someone wants to view the image again, it'll get fetched. I don't know if kbin and lemmy have this feature, but they should. It's probably not the locally uploaded images that'll take up a ton of space, it's caching all the remote ones that will.