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.
PixBuilder - Free Photo Editing Software for PC
Corel Painter - Corel Painter is media painting software.
PhotoDemon - Lightweight portable BSD licensed photo editor for Windows focused on performance and usability.
Fresh Paint - Microsoft's new painting software with more realistic brushes for Windows 10
Fotografix - In addition to being totally portable, Fotografix has a ridiculously small footprint.