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That actually makes it easier:

add_action( 'wpsp_before_header', function() {
    if ( ! has_post_thumbnail() && get_post_meta( get_the_ID(), '_yoast_wpseo_opengraph-image', true ) ) {
        printf(
            '<div class="wp-show-posts-image">
                <a href="%1$s">
                    <img src="%2$s" alt="" />
                </a>
             </div>',
             get_permalink(),
             get_post_meta( get_the_ID(), '_yoast_wpseo_opengraph-image', true )
        );
    }
} );

Let me know 🙂