PixiEditor is a free and open source fully-featured 2D graphics editor, now better than ever with the 2.0 release out that brings Linux support.
The original creator mentioned their mission with it is to "give the world free and open source, offline, Universal 2D Image editor, that can do as much as expensive proprietary creative software such as Adobe's, if not more" and they believe version 2.0 "already outpaces Photoshop in certain areas".

Direct Link
More about it (from GitHub):
Toolsets for any scenario
PixiEditor 2.0 comes by default with 3 toolsets:
- Pixel art - it contains tool suited for pixel-perfect scenarios
- Painting - basic painting tools, soft brushes, anti aliased shapes
- Vector - shapes and paths for creating vectors
All toolsets can be used on one canvas. Mix vector with raster. Export to png, jpg, svg, gif, mp4 and more!
Animations
Version 2.0 comes with a timeline and animation capabilities. You can create frame by frame animations or use nodes to animate your custom shaders. Key frame animations with vectors are on our roadmap.
Nodes
Node render system is what powers such extensive capabilities. All layers, effects and the layer structure are nodes or a result of its connections. PixiEditor exposes node graph for every document, so you are free to customize your image however you want and create procedural art/animations!
Sounds awesome and looks great! Nice to see it get Linux support.
You can find it on GitHub, Steam and the Official Site.