Ahh, but if the artist is alive, you might run into problems if you tried selling prints of your images without a licence. I have to admit, I'm not entirely up on the law with this, not for lack of trying, but I understand that artwork should not take up a substantial part of the picture, although I tend to think street murals are there to be photographed and the artist wouldn't care.

You'll probably fine the galleries have photography policies saying the photos must be only for your own non-commercial use.