Artistry Online is an online resource dedicated to helping artists produce paintings from photographs. It's run by Karen Sperling who wrote the manuals for the first several releases of Corel Painter and who is a wonderful artist as well as a fabulous teacher.
These are the best resources I have found for learning to paint in corel painter and/or photoshop. I have learned more from these books and video then I have from any other resource and I highly recommend you check them out if you are interested in furthering yourself as an artist.
Here is a link to the feature and interview. http://www.artistrymag.com/docs/jackzucker.html