If you have trouble imagining how one might make expressive images using tools like Processing, take a look at the beautiful pixel-style art of Craig @ Superbrothers.
Of course, some of the loveliness of these illustrations comes from the fact that they’re not made of flat, pure-hued pixels—the colored rectangles are grainy, organic, and warm. But the greater part of their success comes, I think, from the concision of the shapes. So much information is conveyed by so few squares.
If you can channel that kind of economy, you can probably draw with anything.