Is it possible to change the face that represents each person on Photos.app on OS X 10.10?
Right-click on the Faces album you'd like modify, and click Get Info. 
Double click the small round photo that appears on the fifth row of the info pop-up. All photos in that album will now appear behind this pop-up.
Right-click the photo you that you want to use as that Faces album thumbnail, and click Make Key Photo.