face.com is a face recognition service.
Unfortunately they only return face positions and not crop information.
Getting the best thumbnails from picture showing people isn’t even trivial when you already know where the heads are. Big pictures with small faces require different margins than small pictures with huge faces.
Various people on one picture are even another challenge.
At work, we’ve implemented an algorithm to find the best thumbnail crop positions using the face.com face detection service. Feel free to use it on your own projects.