private static function findScreenshot($number, $plugin_slug)
    {
        $extensions = array('png', 'jpg', 'jpeg', 'gif');

        // this seems to now be the correct URL, not s.wordpress.org/plugins
        $base_url   = 'https://s.w.org/plugins/' . $plugin_slug . '/';
        $assets_url = 'https://ps.w.org/' . $plugin_slug . '/assets/';

        /* check assets for all extensions first, because if there's a
           gif in the assets directory and a jpg in the base directory,
           the one in the assets directory needs to win.
        */
        foreach (array($assets_url, $base_url) as $prefix_url) {
            foreach ($extensions as $ext) {
                $url = $prefix_url . 'screenshot-' . $number . '.' . $ext;
                if (self::validateUrl($url)) {
                    return $url;
                }
            }
        }

        return false;
    }