Which tool you use to design your graphics won't mean anything if you don't have some artistic ability. I speak from experience. That said, if you decide to g

Which tool you use to design your graphics won't mean anything if you don't have some artistic ability. I speak from experience. That said, if you decide to go 2-D, I would highly recommend a vector graphics editor like Inkscape or Adobe Illustrator. I would argue that vector-based drawing, especially for the less experienced artist, is easier than bitmap drawing. As recommended by J_McQuade, toss some images together at first and really focus on the actual code instead. Once the infrastructure is there, the images can easily be replaced.