After reading this survey, I decided to create one for myself. It worked. But now I can’t get rid of it in order to go back to an avatar-only contact.
Web searches seem to imply that the phone keeps a set of all the posters you’ve created, and you can swipe-up to delete them. But I see no such thing. I can change the poster, but I can’t remove it (nor can I add an additional one.)
This is on iOS 26.1.