Hi Pilou, you might also try using a raster to vector tracer on the background image to trace an outline around a face.

You could bump up the light intensity under Options > View > Lighting options to get solid coloring without shading.

Peer mentioned a couple of different tracers above here:
http://moi3d.com/forum/index.php?webtag=MOI&msg=10114.475

- Michael