Photoscape - high quality photo editing software, that enables you to fix and enhance photos.
Dibuja - Gtk based basic paint program like MSPaint or Paintbrush but for Linux
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.
ACDSee - ACDSee is an image viewer that uses very little system resources, and it has a small, sleek size that won't take up much space on your system.
Affinity Photo - Affinity is the imaging and design suite for creative professionals exclusively for Mac.