You can also block just media if an instance is known to host images that are illegal in your country or your users don't want to see (silencing is a better option for the latter tho).

I believe once you follow someone, you become federated with their instance. and you potentially receive all content from that instance... I think?

there are also admin controls for managing moderation and federation.

I still don't want to host anyone else's content, for much the same reason I'm never going to have an unmoderated comment system on a site.