Ask HN: How does Intercom and other web apps auto populate user images?

Many web apps these days have access to some external data feed where they can check an email address against the feed and see if they can find a headshot for that person. What API are they all using?

Examples I've seen so far include Intercom and AgileCRM.

EDIT:

I'm going to assume that they all just use the Gravatar API.