ok.. figured it out with a bit of googling (being new to gimp). for anyone looking to do it in gimp: 1. open as layer the original photograph 2. open as layer the drawing 3. shift-s to scale drawing roughly 4. make it a multiply layer (layer panel --> mode=multiply) 5. hide original photograph 6. select all (ctrl+A) + copy (ctrl+c) 7. new layer with a foreground colour of red 8. add layer mask 9. paste (ctrl+v) 10. right click --> color --> invert 11. put background picture back on 12. right click --> colour --> levels --> adjust contrast