Gimp
Install

The GNU Image Manipulation Program (GIMP) is suitable for those wanting to edit digital photos, design and export graphics for the web, convert from one file format to another, or even produce complex high-resolution compositions for print. The GIMP features a number of paint tools with sub-pixel sampling for high quality anti-aliased brush strokes, a powerful gradient editor and blend tool, layers and masks with over 20 blending modes, levels and curves tools for enhancement of digital photos and much more! GIMP can also open and save files in wide range of formats.

This application requires the GTK+ toolkit which you should install first. Click to install directly or read more here.