MyPaint is free open-source and simple graphics application for digital painters, works on Linux/Unix operating systems. MyPaint has a clean user interface gives you more space to easily draw and edit images. Also, includes my functionality for such a small and portable applications.
MyPaint designed for pressure sensitive graphics tablets, comes with variant types of drawing brushes, and you still able to add more brushes and modify currently available brushes. Support basic layer. MyPaint includes different types of color selector, you can adjust these colors with keyboard shortcuts.
Kristen Stewart portrait, painted using MyPaint
Check the youtube video shows painting process of Kristen Stewart portrait
Install MyPaint 1.0.0 On Ubuntu 11.10 Oneiric Ocelot Using PPA
sudo apt-get update
sudo apt-get install mypaint
Check MyPaint galleries for more portraits.