These avatars are a feature provided by the used mail client, e.g.:
- GMail will show the user’s Google+ profile avatar if available
- Apple Mail will show the user’s avatar shown in your address book
- etc.
So the avatar is not part of the mail itself.
Two workaround suggestions:
- Use a GMail sender address assigned to your Google+ profile.
- Add a VCard as attachement, see Formatting VCard in PHP and http://www.xmpp.org/extensions/xep-0153.html#publish – so the recipient gets a VCard to import into his address book.
solved PHP mail set from image