I've always wanted to do something like this, but wired up to your webcam to detect when you blink. When you do, update the picture to a new one, to make extremely ephemeral art.
Your actual idea is much cooler.
To expand on your idea, it would be cool if every time you blinked or looked away, it wasn't a completely new picture, but the same one that is subtly changed, such that it slowly morphs over time, but only as you aren't looking. I suppose you could pre-generate quite a few steps in advance and then mete them out gradually.