featured image external link

 <?php
    /*
    Plugin Name: Featured Image External Link from Custom Field in Page
    Plugin URI: n/a
    Description: adds an external link to the 'featured image' in a static page; link url has to be in custom field with the key 'ExternalUrl'.
    Version: 1.0 beta
    Author: alchymyth
    Author URI: http://wordpress.org/support/profile/alchymyth
    License: GPL2

        This program is free software; you can redistribute it and/or modify
        it under the terms of the GNU General Public License, version 2, as
        published by the Free Software Foundation.

        This program is distributed in the hope that it will be useful,
        but WITHOUT ANY WARRANTY; without even the implied warranty of
        MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
        GNU General Public License for more details.

    */

    add_filter('post_thumbnail_html','add_external_link_on_page_post_thumbnail',10);

    function add_external_link_on_page_post_thumbnail( $html ) {
    if( is_page() ) {
            global $post;
            $name = get_post_meta($post->ID, 'ExternalUrl', true);
            if( $name ) {
                    $html = '<a href="' . ( $name ) . '" target="_blank" >' . $html . '</a>';
            }
    }
    return $html;
    }
    ?>