WP: Add thumbnails in Manage Posts/Pages list

gistfile1

/****** Add Thumbnails in Manage Posts/Pages List ******/
if ( !function_exists('AddThumbColumn') && function_exists('add_theme_support') ) {

    // for post and page
    add_theme_support('post-thumbnails', array( 'post', 'page' ) );

    function AddThumbColumn($cols) {

        $cols['thumbnail'] = __('Thumbnail');

        return $cols;
    }

    function AddThumbValue($column_name, $post_id) {

        $width = (int) 35;
        $height = (int) 35;

        if ( 'thumbnail' == $column_name ) {
            // thumbnail of WP 2.9
            $thumbnail_id = get_post_meta( $post_id, '_thumbnail_id', true );
            // image from gallery
            $attachments = get_children( array('post_parent' => $post_id, 'post_type' => 'attachment', 'post_mime_type' => 'image') );
            if ($thumbnail_id)
                $thumb = wp_get_attachment_image( $thumbnail_id, array($width, $height), true );
            elseif ($attachments) {
                foreach ( $attachments as $attachment_id => $attachment ) {
                    $thumb = wp_get_attachment_image( $attachment_id, array($width, $height), true );
                }
            }
                if ( isset($thumb) && $thumb ) {
                    echo $thumb;
                } else {
                    echo __('None');
                }
        }
    }

    // for posts
    add_filter( 'manage_posts_columns', 'AddThumbColumn' );
    add_action( 'manage_posts_custom_column', 'AddThumbValue', 10, 2 );

    // for pages
    add_filter( 'manage_pages_columns', 'AddThumbColumn' );
    add_action( 'manage_pages_custom_column', 'AddThumbValue', 10, 2 );
}