I'd advise that anyone who wishes to write stories about torture should not use pictures of people smilling.

Having said that since we're dealing with women being tortured here it's best not to use any means of torture that leaves physical scars or at least it should be minimized as much as possible.

There's the Chinese water torture.
Or maybe you could tie one of these girls up in an uncomfortable position for extended periods of time.
Sensory deprevtion.