This is just a really quick and easy way of doing mountains (about 30 minutes tops for a whole mountainoues background), I recommend adding more detail alter, like snow or trees or something.

1. Draw in the basic outline of your mountain. I'm doing a triangle because that is one of the simpler shapes.

2. Decide which angle the light will hit it. On the side that will be highlighted, outline it in a lighter color. On the side that will be shadowed, outline it ina darker color. Also, add ridges or large grooves now. (I've done on going straight down the middle.)

3. Using the smudge tool, drag down the colors to the center of thier side, and you come out with a finished result. Ta-da, whoop-dee-doo.

You can add snow by outlining the top in white and blurring the color, and trees by adding in green smudges. This same technique is also good for sand dunes.