Image missing and required - Wordpress AMP Structure doesn't add Image attribute

When validating my wordpress posts using Google's Structured Data Testing Tool, I get the following error:

"Image: missing and required"

I have the official wordpress AMP plugin installed that generated the AMP pages for me. The problem is that it doesn't popular the "image" attribute for BlogPosting.

In the plugin there is a code that I think should generate it, but it's not running anywhere:

private function get_post_image_metadata() {
    $post_image_meta = null;
    $post_image_id = false;

    if ( has_post_thumbnail( $this->ID ) ) {
        $post_image_id = get_post_thumbnail_id( $this->ID );
    } else {
        $attached_image_ids = get_posts( array(
            'post_parent' => $this->ID,
            'post_type' => 'attachment',
            'post_mime_type' => 'image',
            'posts_per_page' => 1,
            'orderby' => 'menu_order',
            'order' => 'ASC',
            'fields' => 'ids',
            'suppress_filters' => false,
        ) );

        if ( ! empty( $attached_image_ids ) ) {
            $post_image_id = array_shift( $attached_image_ids );
        }
    }

    if ( ! $post_image_id ) {
        return false;
    }

    $post_image_src = wp_get_attachment_image_src( $post_image_id, 'full' );

    if ( is_array( $post_image_src ) ) {
        $post_image_meta = array(
            '@type' => 'ImageObject',
            'url' => $post_image_src[0],
            'width' => $post_image_src[1],
            'height' => $post_image_src[2],
        );
    }

    return $post_image_meta;
}

How can I populate the image tag for each post using this AMP Wordpress plugin? I want the page to pass the Sturctured Data Testing Tool, so he can pass the AMP validation as well.

via Chebli Mohamed