GIMP, or the GNU Image Manipulation Program to give it its proper name, is a graphics-editing program broadly similar to Adobe Photoshop in terms of functionality. The GIMP is an open source program that can be freely downloaded and installed on most computers, including maps. But on the downside it doesn’t come with a manual, so figuring out how to use GIMP can be tricky. Read the full review here.
The Artist’s Guide to GIMP Effects