This a class activity I've been doing for a number of years at BCU and Glamorgan uni. At BCU i used to use two portraits of myself and the other tutor on the module, but i've now taken it further by allowing the students to mix and alter the faces of themselves and their class mates which causes much laughter as they mix eyes and noses etc.
You could also try images of celebrities or politicians.
the work of martin schoeller would work, as the composition and lighting is consistent.
I also show them various projects done by artists who have used this or a similar technique.
the work of genetic.ulriccollette.com/ is one recent example.
I hope you will have a go at trying this out.