In about two hours I will be giving my Design to Theme (in five simple steps) presentation at CMSExpo in Chicago. This one-hour presentation covers the basics of how to convert a design file into a Drupal theme. I use GIMP but the lessons work for Photoshop (and Inkscape and Illustrator and and and) too.

If you're interested in learning more there's also an ebook and an online class starting May 11.