You can change the mode to Black and White

Suzanne — Mon, 28 May 2001 22:29:06 +0000

And choose dithering options, but that won't give you entirely what you are looking for, I think. It might, though.

Otherwise, you can desaturate the entire piece and pump up the contrast until you get them all separated out into black and white.

Suzanne

Werks — Mon, 28 May 2001 15:57:49 +0000

just black and white

Mark Hensler — Mon, 28 May 2001 06:25:03 +0000

just black and white, or grayscale?

to make it grayscale, I think its:
Image > Mode > Grayscale