GIMP - GIMP is a multiplatform photo manipulation tool.
Pixelmator - Pixelmator is an image-editing application for Mac and iPad.
Krita - Krita is a professional FREE and open source painting program. It is made by artists that want to seaffordable art tools for everyone. Concept art. texture and matte painters, illustrations and comics.
Photoscape - high quality photo editing software, that enables you to fix and enhance photos.
Affinity Photo - Affinity is the imaging and design suite for creative professionals exclusively for Mac.
Pixlr - Pixlr® makes it easy to transform everyday images into stunning works of art.