Try Gimp at www.gimp.org
It's an open source graphics creation and manipulation program that rivals Photoshop. It takes a while to learn to use it well, but compared to other similar programs is just as user friendly as the best of them.
It has a cartoon and posterize filter that let you take existing pictures (that you've drawn of even photos) and make them look like cartoons. Look through the tutorials and do online searches for gimp tutorials. I remember seeing one somewhere about taking a photo or drawing and creating a 16 bit version of it (that is to say old school graphics like the original NES).