The Poser 4 Handbook by Mortimer Shams (not sure if the spelling of the author's last name is correct) is a good 3rd party book that cover things in more detail about texture creation than the Poser Manual. Although a lot of people don't like this book, as a Poser newbie it has been of great help to me. It has tutorials that specifically cover what your interested in. Although the author uses Painter in the tutorials, the steps can be applied to any image editing software that allows for layering and selection capabilities.