It is not compulsory for members to upload face pictures to their Private Gallery, but quite a good way to earn some income if you don't want everyone seeing your face in your free gallery.
If you want to be included in the new search, when uploading images to your Private Gallery if you start the picture title with *f* it will be flagged as a picture showing your face.
If you are uploading via iOS, start titles with fp if your face is visible.
You can also change the setting on existing photos via Manage Private Pictures.
Note: Even when set as a Face Picture, the FP will not appear until one of the moderation teams has confirmed it. This can take anything from 20 minutes to several hours.