Script-Fu -> Inkpen
This is an adjustable script which renders the original image as if drawn with an ink pen. The settings in the dialog made can make a huge diference in the results. Also a major consideration is the light/color contrast in the original image. Perhaps the most important setting is the "brightness", but ink color as well as the stroke length are also adjustable, and there are toggles for making outlines bolder and creating a blue background grid.
The examples will illustrate this...
"the Lovers" by Pablo Picasso
Brightness: -79 Stroke length:30 No "Outline" No "Grid"
Brightness: -79 Stroke length:10 No "Outline" No "Grid"
Brightness: -79 Stroke length:30 "Outline" No "Grid"
Brightness: -79 Stroke length:30 "Outline" "Grid"