DISPLAY POST THUMB ON EDIT POSTS SCREEN (WORDPRESS)

/* DISPLAY POST THUMB ON EDIT POSTS SCREEN */
if(!function_exists('displayThumbsColumn') && function_exists('add_theme_support')) {
    function addThumbsColumn($cols) {
        $cols['thumbnail'] = __('Post thumbnail');
        return $cols;
    }
function displayThumbsColumn($column_name, $post_id) {
    switch($column_name) {
        case 'thumbnail' :
            $thumb = get_post_meta($post_id, '_thumbnail_id', true);
            if (empty($thumb)) break;
            else echo wp_get_attachment_image($thumb, 'thumbnail');
            break;
        default :
            break;
        }
    }
// display on posts edit screen
add_filter( 'manage_posts_columns', 'addThumbsColumn' );
add_action( 'manage_posts_custom_column', 'displayThumbsColumn', 10, 2 );
// display on custom post type (named "my_custom_post_type" for clarity...) edit screen
add_filter( 'manage_edit-my_custom_post_type_columns', 'addThumbsColumn' );
add_action( 'manage_my_custom_post_type_posts_custom_column', 'displayThumbsColumn', 10, 2 );
}