I've only used this a few times because I'm not an artist however I always like to have multiple image programs on hand because they each have some feature the others don't. Gimp is pretty advanced and definitely worth the download; especially if your sitting there using paint or some nonsense. Here's a quote from there site:
GIMP is an acronym for GNU Image Manipulation Program. It is a freely distributed program for such tasks as photo retouching, image composition and image authoring. It has many capabilities. It can be used as a simple paint program, an expert quality photo retouching program, an online batch processing system, a mass production image renderer, an image format converter, etc.Get GIMP here.