The issue doesn't seem to be the Fediverse itself, rather the fact that images uploaded to Lemmy are handled in a separate program that isn't linked to it in a way you can delete from by just deleting posts. The images aren't marked as owned by you, so can't be deleted again. You'd need some way of storing those image deletion tokens against your account, so you can manage them yourself and be able to delete them again.
And this would have to include images that you uploaded and didn't make a post about. As far as I can tell they're just left there on the server forever. Not even sure if it tells you which user uploaded it, although it might log by IP address. I haven't looked too deeply into the code but there's potential for abuse there.