MyPaintBasic tutorialThe greatest free painting software out there.
You can read about it on the website, or its wiki.
But in case you're too lazy to click the link, here's a basic FAQ thingy.
Q: Is free????
A: Yes, I just said it was.
Q: How 2 use???
A: MyPaint is 70% controlled by the keys on your keyboard, you can change them to your liking if you want.
Q: Does has layers??
A: Yes, it has basic layer support.
Q: How to re-size canvas???
A: The canvas unlimited, you don't have to.
Q: Is pressure sensitive with a drawing pad??
A: Yuppers.
That's about it. One more thing, you can download extra brushes in case you don't like the ones that come with it.
You can also do the same with backgrounds.
This program is purely for drawing and painting, it's not meant for image editing.