Printing in black and white without intermediate greys
To convert easily any image in Black &amp; White (no gray):


Open the file in Preview
Choose "Save As…" (Visible in the File menu, while holding the Alt key in Mountain Lion)
In the save dialog, choose the PDF file format
Still in the dialog, apply Quartz filter: "Black &amp; White"




EDIT: You can convert to black &amp; white with dithering using The Gimp (free &amp; open-source):


Open the image using The Gimp
Select menu item Image -> Mode -> Indexed…
On the window that opens:


Select the Black &amp; White palette
Select a dithering mode (I chose Floyd-Steinberg)



You can also use Photoshop using the same technique. However, Photoshop is expensive.

Finally, command-line solutions exist: convert (part of the imagemagick package), or gm (part of the graphicsmagick package).