I think this might be a bug. A user just told me about that and i can confirm it with my testusers. I have a category with adult material. Access only by a usergroup 18+. This works well. But if i go over the media in a users profilpage or card and click on his number of media i can see this pictures in the gallery like all other thumbnails doesn`t matter in what usergroup i am in. Clicking on this thumb give a error. Don`t have permission to see it. So in my opinion they should also not show up as thumbnails. This is a real big problem. I must shut down the gallery for this.