Simple to use paint program like Paintbrush for Mac or classic MS Paint, but for Linux.
Photoflare is a cross-platform image editor with an aim to balance between powerful features and a very friendly graphical user interface. It suits a wide variety of different tasks and users who value a more nimble workflow. Features include basic image editing capabilities, paint brushes, image filters, colour adjustments and more advanced features such as Batch image processing.
Krita is a free and open source painting tool designed for concept artists, illustrators, matte and texture artists, and the VFX industry.