Being an artist myself, you're a lot better off actually getting a physical model to pose for you. It may sound awkward at first, but using photos doesn't really force your brain into learning mode to understand depth and proportion in real time.
While these photos are amazing for reference on gear and such (I just saved a ton) you're better off finding a friend to do brief poses for you to scribble down. (Give him a gun and kaboom)
Another good site for reference on anatomy and action poses is
Http://Posemaniacs.com/
While it is still a 2d image, the images are actually interactive, and you can rotate the pose to different angles if it permits.
Overall, Physical models work best, your brain learns better this way.
Not really in context to ASC (Warhammer fans will know what it is!) but this is some of the work I've done, really basic understanding of anatomy and such.
Good luck!