Hi Wolf great to have you here on the forums...this all depends on what you are planning to do with the piece.
First, how close to the photo is the artwork and could it be recolonized to the photo easily? If not there should not be that big of deal...although all artists like to be recognized for their work. Second, where did you get the photo? Was it from a photo morgue site of a random photo? You may not have top credit the source in some cases because the photo was meant to be used.
The best thing to do at any given situation is to get permission from the photographer beforehand and tell hem what you plans are for the artwork.
Okay, what is your plans for this artwork? Do you plan on making prints or just selling it outright? If so and your piece is recognizable compared to the original you could run into problems...I wouldn't make prints of this piece but I would sell it outright without any real concerns.