This came up on https://wordpress.org/support/topic/image-amp/

When a blog post has an image added to the content but does not have any image assigned as the featured image, then GSC will complain about there not being an image supplied. We should automatically scrape the post_content for the first image to use in this case.

Note that Jetpack actually has some similar logic to do this in filtering amp_post_template_metadata: https://github.com/Automattic/jetpack/blob/c126df3d52a9e7e35cb1cefcf64281448d8becd9/3rd-party/class.jetpack-amp-support.php#L129-L174

As with #1812, we should only use an image if it satisfies the required dimensions.