Literally, in this case.
I’ve been fiddling around in Gimp lately, and this is what I came up with. Gimp (which stands for GNU Image Manipulation Program) is a powerful and popular photo-editing program. You can download it for free here. Isn’t freeware great? Gimp is very powerful, and can do things from creating images to extensively editing existing images. It has a plethora of brushes, filters, and plug-ins. For example, the sun in this picture was generated by a plug-in. I just had to enter the size and color.
This image, aside from a few stock textures for the planets, was created entirely in Gimp.
Gimp isn’t that intuitive, but once you become familiar with the interface, you can do practically anything. To get used to the controls, I’d recommend going through a few internet tutorials.
Suffice to say, I really like Gimp. And the $0.00 you pay for it makes it even better.