From Wikigogy

Gimp is a popular free open source computer painting tool similar to Adobe Photoshop in purpose. User:Roger uses Gimp to manipulate and adapt clip art for lesson materials.

See http://gimp.org to read more or download it.

Inkscape on the other hand is a popular free open source computer drawing tool (vector drawing tool) similar in purpose to Adobe Illustrator but not as fancy, and free. User:Roger likes to use Incscape to manipulate and adapt clip art for lesson materials when the source clip art is in .svg vector file format which is easy to manipulate and scale.