The Gimp, or GNU Image Manipulation Program, is an excellent, free image editing software package for Linux, and Windows. As far as I can tell, it does everything the big name programs do, except allow the creation of shapes. Coming from Fireworks, it was tough to get used to. But you can create vectors and areas, and then fill them. While I am not a professional, I did create this graphic using The Gimp.
It does take a bit to learn, but once you do you'll be glad you did.