The solution is in their GitHub docs: https://github.com/Automattic/amp-wp
Look for the “Schema.org (JSON) Metadata” section, it has a demo.
I can see why they didn’t want to make this automatic (the formatting guidelines are extremely precise: https://developers.google.com/search/docs/data-types/articles) but as-is it’s a huge impediment to the “turn on and it works” theme of the plugin. Anyone who doesn’t implement that code snippet to define their image is going to miss out on the carousel thing in google results that is the reason why AMP has become so mandatory :S
