What email provider and/or client is he using? What type of device? If he's using a mobile device, maybe it's pulling an image from his contacts.
These articles explain other possibilites.
https://askleo.com/why-is-my-picture-showing-up-next-to-emails-i-send/
https://www.theinternetpatrol.com/warning-having-email-display-senders-contact-image-and-info-helps-scammers-get-in-through-the-cracks/
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