Pixia is the English version of a popular free Japanese painting and retouching software for full-color graphics.
It supports masks, layers, and many other editing functions. You can use your customized brush tips and even use an image file as a brush tip to create unique effects.
Pixia's user-friendly interface is suitable for beginners as well as experts. And it supports the most common editing techniques that will prove invaluable for anyone working within the graphic medium.