The user picture is provided by a field called user_picture.
If you perform a minimal installation of Drupal 8, this field is not created, so you have no user pictures what so ever. The "User pictures in posts" and "User pictures in comments" options in the theme settings are disabled as well.
If you create a new image field, there is no way to make this field the "user pricture" field. And due to the special naming of the "user_picture" field (instead of eg "field_user_picture") there seems to be no way to achieve this through the interface.