Some services use Gravatar[1] or Libravatar[2] but a user has to first register their email with those services.<p>[1] <a href="https://en.gravatar.com/" rel="nofollow">https://en.gravatar.com/</a><p>[2] <a href="https://www.libravatar.org/" rel="nofollow">https://www.libravatar.org/</a>