...I just used the 4-bit grayscale (16-bit) .aco file from this webpage:
http://notlost.blogspot.com/2008/05/...pths-with.html
and then converted the image to indexed color using that palatte. That way you can use photoshop's dithering methods (pattern, noise, etc.) and save a gif/png which is going to be 1:1 exactly what you'll see on the screen.