Simple & versatile image editor.
Drawpile
Drawpile is a Free software collaborative drawing program that allows multiple users to sketch on the same canvas simultaneously.
Drawing
This application is a basic image editor, similar to Microsoft Paint, but aiming at the GNOME desktop.
My Paint
MyPaint is a nimble, distraction-free, and easy tool for digital painters.
Krita
Krita is a FREE and open source painting tool designed for concept artists, illustrators, matte and texture artists, and the VFX industry.