Were I to guess I'd say he made the image a background layer then used the pen/vector tool to lay down the lines on top of that image as the groundwork then possibly blurred the original for the colors. Some editing was probably done where necessary. The lightning was probably improvised. If that was the case, then I really think you should say that you more than referenced a pic. Because when I say I reference a pic I mean that I use a photo for pose refinement, color adaptation, or some other contextual necessity: style, clothing and so on. It's used for parts or sections of a work, not for an indirect copy of that source picture. There's nothing wrong with using a picture in that manner, provided you give the proper credit.