Gimp is an acronym for the GNU Image Manipulation Program.
It is a free, open source raster image editor available on multiple operating platforms (Windows, Linux, OS X, etc.)
Gimp features a customizable interface, photo enhancement and digital retouching features, as well as support for multiple hardware devices and file formats.
It is expandable by way of plugins and extensions that can be added or created via its scripting interface.
As a cross-platform image-editor, Gimp is unparalleled.