GIMP (short for GNU Image Manipulation Program) is a free, open-source image editor with advanced features, such as layers, automation using scripts, paths and masks. GIMP runs on Windows, MacOS, Linux, BSD and Solaris systems.
GIMP is not used commonly by mappers due to its steep learning curve and because there are paid alternatives (e.g. Photoshop) which provide similar features with a more accessible user interface. However, it is used by some mappers, who have often learned the tool for other uses, such as for school.