If you still have access to a scanner, you can have some fun playing with this idea. Take a piece of paper or a white card and paint on it—I mean with real paint (or great big markers). Scan it in (after it’s dry of course) and use that as a shape in a layer mask. (Hint: Scan it at a higher resolution than you think you’ll need so you have some room to play with resizing.)
Here’s the result of my scan.
Take a look at your scan and see what you’ve got. ...