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   /* Display post thumbnail meta box including description */
    add_filter('admin_post_thumbnail_html', 'plc_post_thumbnail_add_description', 10, 2);

    function plc_post_thumbnail_add_description($content, $post_id) {
        $post = get_post($post_id);
        $post_type = $post->post_type;       
        if ($post_type == 'post') {
            $content .= "<p><strong><label for=\"html\">The Featured Image should be at least 1166px wide and 480px tall</label></strong></p>";
        }
        return $content;
        return $post_id;
    }