How does Photo Caller ID work?

Sometimes someone will call me and their photo will show as the phone rings. In the cases I'm referring to I've never added a photo to the contact---somehow the phone picks up their image over the Internet. I'd like to register my photo in such a place, but how does it work? I have had both iPhone and Android users call me and sometimes I get their "Photo Caller ID" for lack of a better term. My phone is an HTC m8 running LineageOS 14.1. Thanks for your help!