This example was using Gimp 1.2
I got this tip from the Gimp User Manual on the Gimp Homepage, in the section "Gimping Around In 80 Minutes".
I assume you already have a picture with your person-sans-background.
- Make yourself a new white layer below the person, and merge the layers.
- Desaturate the layer with Image -> Colors -> Desaturate.
- Reduce the number of colours with Image -> Colors -> Posterize. If you make it too few, then your person won't have any features left; if you make it too many, then it won't look like a sketch.
- Make yourself a new white layer above the person layer, and set it to Screen mode.
- Make this the active layer, and temporarily make it semi-transparent, so that you can see the image underneath.
- Because black is transparent in Screen mode (also Addition and Lighten Only) black pencil strokes drawn on a white layer will reveal the image underneath, just as if you had sketched all by hand. Of course, unless you have a tablet, it's a bit harder to control a mouse than a pencil, so it may end up looking like something drawn by a demented four-year-old. Practice, practice, practice.
- When you've scribbled to your satisfaction, make the Screen layer fully opaque, and do Layers -> Merge Visible Layers.
You could also get different effects by using different brushes, or maybe using the Airbrush tool. You can also apply effects afterwards, like Filter -> Artistic -> Apply Canvas.