Pinta is ideal for basic image editing. For beginners it’s quite simple to use.
Krita is an advanced painting program, which comes with HDR and PSD support. Also, it brings stability to shaky brush strokes..
A paint application cannot get much simpler which a 3 year old can also use. It can be used for introductory computer classes
With Drawpile multiple artists can work on their art projects simultaneously and can chat with each other. You can also record the drawing sessions.
Mypaint is a simple drawing tool with powerful features. It’s key feature is minimal interface and distraction free drawing board for more focus.