Gnu Image Manipulation Program: Free and Open Source Software for image creation and editing. It can also be used for animation. Gimp is a graphic application equivalent of Adobe Photoshop. GIMP is suitable for those wanting to manipulate digital photographs, design and export graphics for the web, convert from one file format to another, or even produce complex high-resolution compositions for print. Tutorials are available on the GIMP website http://www.gimp.org

It is available for both Windows and Linux platforms.