Krita

Targeted at digital painting and animation purposes, Krita is the best option. It's a painting design app for illustrators, concept artists, comic book creators, matte painters, and texture artists, and the VFX industry. Krita is used daily by professional artists all over the world, as well as enthusiastic amateurs. With translations in many languages, you can create artwork without any obstruct.

Version : 4.0.x (2016-04-30)
Better text tools and vector art capability, python scripting support, etc.

Version : 3.0.x (2015-04-30)
Better performance, animation support, layer, workflow, transform, filter, brush, etc.

Version : 2.9.x (2014-06-30)
Faster Development, better PSD support, layers, masks, brush, resource manager, display, etc.