GIMP - GIMP is a multiplatform photo manipulation tool.
digiKam - Professional Photo Management with the Power of Open Source
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.
XnView MP - XnView is a free software that allows you to view, resize and edit your images. It supports more than 500 different formats!
MyPaint - MyPaint is a fast, distraction-free, and easy painting tool for digital artists.
FastStone Image Viewer - FastStone Image Viewer is a fast, stable, user-friendly image browser, converter and editor.