Cool image! Looks kinda like the Posterize Filter in Photoshop. Scanner Darly had fewer colors for gradation - try lowering the colors from 36 to 12 and see what happens?

Linklater hand-traced each frame of the movie (it was shot with only a Panasonic HVX100 if you wanna try it yourself) - and while i think there was custom software involved, he controlled the thickness of the lines for a cartoon-like look.

Cool start with the image, though - maybe even try lowering it to 8 - really get some thick banding going and it might make your tracing of lines easier to do to control thickness?

Good luck-

-Lew ;-)