For those who remember about my post on Ubuntu, the best art of operating system crafted... you should remember that I do mention about GIMP - The GNU Image Manipulation Program.
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.
GIMP is expandable and extensible. It is designed to be augmented with plug-ins and extensions to do just about anything. The advanced scripting interface allows everything from the simplest task to the most complex image manipulation procedures to be easily scripted.
GIMP is written and developed under X11 on UNIX platforms. But basically the same code also runs on MS Windows and Mac OS X.
You can read more on Official GIMP Website...
For me, it is the best replacement for Adobe Photoshop and it's free!! Take a look here on how this guy enjoy painting using GIMP.
You have choice now, if you are using those licensed Adobe Photoshop, yes it's also good but if you don't have any licensed photo retouching or painting tool, grab gimp now. And start producing your art in GIMP... Enjoy!!