GIMP-GNU Image Manipulation Program
GIMP (GNU Image Manipulation Program) is an image retouching and editing tool and is released under the LGPLv3 and later versions as free and open-source software. There are versions tailored for most operating systems including Linux, OS X, and Microsoft Windows.
The above all stuff is form GIMP wiki.
in my words GIMP is amazing piece of work.
you can download GIMP from the links below for Linux as well as for Windows.
For more information go to home page.