The Free & Open Source Image Editor
The official website.
GIMP is a cross-platform image editor available for GNU/Linux, OS X, Windows and more operating systems. It is free software, you can change its source code and distribute your changes.
Whether you are a graphic designer, photographer, illustrator, or scientist, GIMP provides you with sophisticated tools to get your job done. You can further enhance your productivity with GIMP thanks to many customization options and 3rd party plugins.
Download the software.
HERE you will find some useful tutorials on how to use GIMP software.
Awesomebump ( free texture map creation )