Please allow me to explain by means of analogy: say I'm an illustrator working for Marvel ( kind of a stretch, but just play along, OK?). They've contacted me to do a cover for an issue of Spider Man. I do my thumbnails, I decide on a composition that'll look good, and I go in to do the final piece. For the sake of realism, I decide to use some photos as reference. I go online and find a picture of a model posing in a similar pose to what I'm trying to accomplish for my cover.
So, I download it and refer to the image as I'm doing the underdrawing for the final piece. Bear in mind that this is not a photomanipulation or a collage: no part of the original image is being used in my cover art and no attempt is being made to replicate or even be derivative of the photo in question. The medium has been changed, and the image I've created is an original work (well, aside from the fact that I'm legally depicting a character that someone else created 46 years ago). Do I bear any responsibility, legal or otherwise, to the photographer who snapped the photo? The only use I've made of his or her work is as visual inspiration for how physical bodies behave in the real world.
Would this be considered Fair Use? Any good resources for legal precedents or resources on copyright law which might clarify this point? Thanks in advance.