Seems this is a tough one to do with any sort of thematic detail on the map. If the map is in colour, you have to convert it from RGB to greyscale before you can change the display properties to classified, so you lose some detial there. Then when you lower the number of classes to two, you lose lots more, particularly if there are thin contour lines. I tried selecting the contours in GIMP and then putting them in a seperate image, but the requirement for two classes only still caused problems.
Any suggestions?