amp-schemaorg-meta-image-content-fallback.php

<?php
/**
 * Plugin Name: AMP Schema.org Meta Image Content Fallback
 * Description: Promote content image as Schema.org image when no featured image is supplied.
 * Plugin URI: https://gist.github.com/westonruter/7358d6669e09cfa15a99021204208a67
 * Author: Weston Ruter
 * Author URI: https://weston.ruter.net/
 * Gist Plugin URI: https://gist.github.com/westonruter/7358d6669e09cfa15a99021204208a67
 */

add_filter(
	'amp_schemaorg_metadata',
	function( $data ) {
		if ( ! is_singular() || ! empty( $data['image'] ) || ! get_post() ) {
			return $data;
		}

		if ( preg_match_all( '#<img\s[^>]+(?:\s|"|\')wp-image-(\d+)(?:\s|"|\')[^>]*>#s', get_post()->post_content, $matches ) ) {
			$attachment_ids = $matches[1];

			foreach ( $attachment_ids as $attachment_id ) {
				$src = wp_get_attachment_image_src( $attachment_id, 'full' );
				if ( ! $src ) {
					continue;
				}
				list( $url, $width ) = $src;
				if ( $width > 1200 ) {
					$data['image'] = array(
						'@type' => 'ImageObject',
						'url'   => $url,
					);
					break;
				}
			}
		} elseif ( preg_match( '#<img\s[^>]*src=(?:"|\')(.+?)(?:"|\')#s', get_post()->post_content, $matches ) ) {
			$data['image'] = array(
				'@type' => 'ImageObject',
				'url'   => $matches[1],
			);
		}

		return $data;
	},
	100
);

Installation instructions: https://gist.github.com/westonruter/6110fbc4bef0c4b8c021a112012f7e9c