Ok, so after giving this some thought, I've decided to add a separate permission for media only.
Reason being that I appreciate that in some contexts the media that can be added to a gallery can be potentially more risque than written content, and more care might need to be taken over this content. Further, we did actually have a separate permission in XFMG1, so it seems reasonable to bring it back.
Finally, the permission itself can be revoked on a per category basis, therefore making it so that certain categories must have their media moderated whereas other categories can be freely posted in (and vice versa).