Cacher is the code snippet organizer for pro developers

// add the namespace to the RSS opening element
function grav_add_media_namespace() {
  echo 'xmlns:media="http://search.yahoo.com/mrss/"\n';
}
add_action( 'rss2_ns', 'grav_add_media_namespace' );

function grav_add_featured_image_in_rss() {
    $featured_image = false;
    if ( has_post_thumbnail() ) {
        $featured_image = wp_get_attachment_image_src( get_post_thumbnail_id(), 'post-thumbnail' );
    }
    if ( false !== $featured_image  ) {
        echo "\t" . '<media:content url="' . $featured_image[0] . '" medium="image" />' . "\n";
    }
    
}
add_action( 'rss2_item', 'grav_add_featured_image_in_rss' );