Filter WP SEO's OpenGraph and Twitter image meta tags

functions.php

<?php
/**
 * Return 'medium' image for mobile devices since that is what we're 
 * loading to display as the featured image on each post.
 */

//* Filter WP SEO's OpenGraph image output
add_filter('wpseo_opengraph_image_size', 'disi_opengraph_image_size');
function disi_opengraph_image_size($val) {
	if( wp_is_mobile() ) {
		return 'medium';
	} else {
		return 'large';
	}
}

//* Filter WP SEO's Twitter Card image output
add_filter( 'wpseo_twitter_image_size', 'disi_custom_twitter_image_size');
function disi_custom_twitter_image_size( $img_size ) {
	if( wp_is_mobile() ) {
		return 'medium';
	} else {
		return 'large';
	}
}