Yep, it's against AA/US policy (UA has one as well, not sure about others) to take photos/video with other employees/passengers in them. Absolutely nothing illegal about it at all, sound like you got a powertripper. Sounds like you did everything right to making sure no one was in the photo. My favorite are the powertripping airline employees going after people taking pictures of the aircraft, their empty seat, etc. Don't they have more important things to worry about?