David90, a thousand thanks. I have spent about six hours trying to solve my problem (after changing themes, my home page was replaced by an Archives page, which had been my blog page). I would never have looked in Settings>reading and noticed the “latest posts” box clicked. You the man!
Thank you, James. I really appreciate your quick reply. I will give this a go this afternoon.
Thank you, Urosino. I have looked over the page you suggested. I’m no expert coder, but as far as I can tell, this method permanently replaces the avatar. I’m looking for a way to override one person’s default avatar now and then, without changing that default. If your link has the answer, my apologies.