If I am starting from a photo, I tend to use photoshop.
Additional to the resources on this forum and the blogspot, I managed to locate a number of tutorials out there.
I'm by no means a "scroller expert", but have used photoshop for a good number of years.
Basic principle is get the image to a nice DPI (dots per inch, say 300). Convert to Grayscale. This will show you the basics of how the light and dark will progress into your final image.
Increase the brightness and contrast to further enhance the image.
2 filters can then be used (in photoshop) Photocopy and Stamp. using various differing settings within these filters give you a starting point. This then needs to have time spent adding bridges and removing islands where practical.
One specific tutorial for photos I found recently is
http://www.80artdesigns.com/photoshop1.htm