You may well own the photo but not the subject of the photo. I believe this means that you can not sell the photo without also acquiring a license to the subject.

I imagine that this is the same as taking a photo of a stock image. That doesn't make any sense.

Reminds me of why you can't take a photo of the Eiffel Tower at night. https://youtu.be/M16CGK1T9MM

Most often yes, but it depends on the gallery. The ones that have the big "no photography" signs are a giveaway.