Replace default og:image tag YOAST

Use the wpseo_opengraph_image_size filter. You can use any WP image size (default ones like medium or ones added by themes and plugins which will be specific to your site) in the first line:

// set 'medium' to whichever size you need
$size="medium";
// the size is then applied in this filter
add_filter('wpseo_opengraph_image_size', create_function("$size", 'return "opengraph";'));