Yeah, it’s only communities that people on your instance search for/subscribe to afaik. So if you’re the only one on your instance then you have control over that.
Further to that it’s only the post objects (and comments, etc.) that is replicated all pics and videos are just URLs. even when you upload a picture with the post, that’s just uploaded to the instance and the link to it is the link of the post, even on other instances the images are fetched from the original source from the client side. I do believe each instance does local thumbnaling.