Once connected to your app, all you need is to grab their user id. You can get this by callinghttps://graph.facebook.com/me (or parsing the access_token Facebook gave you). To get the users friends photos, you will want to access /me/friends and grab the id’s. The image you are most likely looking for is this one, as it is their face and always a small squre:
https://graph.facebook.com/4/picture?type=square
The other images will all be variable sizes across users:
https://graph.facebook.com/4/picture?type=small
https://graph.facebook.com/4/picture?type=normal
https://graph.facebook.com/4/picture?type=large
