Relevant Stack Overflow: http://stackoverflow.com/questions/35710385/image-missing-and-required-wordpress-amp-structure-doesnt-add-image-attribute
I'm in the same situation. Thousands of posts on a site where I'm enabling AMP. Most of back-catalog has missing "featured image" in WP because the feature didn't exist at the time AND because some posts just don't have images (also applies to some new posts, though most have images because we make it a publishing condition now). Whenever there's no featured image the WP plugin does the right thing and leaves the 'image' field empty.
From what I can tell, this is acceptable from a schema.org perspective, and isn't an "error". Correctly, Webmaster Tools doesn't treat a missing image as an "error".
What it DOES do is treat it as an "issue" of type "Info: Invalid structured data element", and lists it along with errors in the AMP section of Webmaster Tools.
I want to get rid of these warnings, and it would be pretty easy to do so by setting a default image to go in that field of the structured data. This is exactly how we use Open Graph's image field for Facebook and friends, so our logo shows on posts with no image.
IS THAT ACCEPTABLE IN THE CONTEXT OF AMP?
That's the question I find unclear when reading the docs: https://developers.google.com/search/docs/data-types/articles
It says "The representative image of the article. Only a marked-up image that directly belongs to the article should be specified." but this doesn't clarify my question, nor does it express how warnings will constantly show if you fail to specify an image.
Obviously if I set a default image I risk people seeing our logo in the carousel and finding it useless, but from our perspective this is still better than not being shown at all. On the other hand if Google is going to PUNISH us somehow for re-using a generic image then it wouldn't be worth it at all.
Any clarity here would be very useful, a clarification in the docs would be even better.
Thanks for any feedback,