function thumbnail_shortcode($atts) {
  $atts = shortcode_atts(array(
    'size' => 'thumbnail',
    'class' => 'wp-thumbnail'
  ), $atts);

  if (has_post_thumbnail()) {
    return get_the_post_thumbnail(null, $atts【'size'】, array('class' => $atts【'class'】));
  }
  return '';
}
add_shortcode('thumbnail', 'thumbnail_shortcode');

function advanced_thumbnail_shortcode($atts) {
  $atts = shortcode_atts(array(
    'size' => 'medium',
    'default' => get_template_directory_uri().'/images/default.jpg'
  ), $atts);

  if (has_post_thumbnail()) {
    return get_the_post_thumbnail(null, $atts【'size'】);
  } else {
    return '';
  }
}
add_shortcode('adv_thumbnail', 'advanced_thumbnail_shortcode');