Re: citation, where do i put it?
I had a problem with this. I made a painting from a photograph for a client, then after the painting was done, I was told that the photo had actually been done by a professional photographer and that it was not the client's work. I was inadvertently copying someone else's copyrighted work.
I have never accepted another "paint from photo" job. I think that the artist needs to go to the place, and take the photos him/herself.
As to your question, I would just annotate, perhaps in pencil on the back, what the source is. (Your name-- from a photo by --Photographer's name) This is also a good place to put other identifying information such as: (San Francisco, 2017).