replace_og_image

//Replace og:image with either the post featured image, or an image of the speaker
//Requires Yoast

add_action( 'wpseo_opengraph', 'replace_og_image', 30 );
function replace_og_image() {

	global $post;

	$featured_image = wp_get_attachment_image_src(get_post_thumbnail_id($post->ID), 'full');
	$speaker = get_field('speaker', $post->ID);
	$speaker_image = wp_get_attachment_image_src(get_post_thumbnail_id($speaker['0']->ID), 'full');

	if ( $featured_image['0'] ) {
		$og_image = $featured_image['0'];
	} else if ( $speaker_image['0'] ) {
		$og_image = $speaker_image['0'];
	}

	if ( $og_image ) {
		add_filter('wpseo_opengraph_image', function() use($og_image){
			return $og_image;
		});
	}
}