The change of data in controller doesn't update automatically

Not only that, it means this, bye bye self variable :

// var self = this; No longer needed with fat arrow functions
return new Promise((resolve, reject) => {
  window.imagePicker.getPictures(function(results) {
    for (var i = 0; i < results.length; i++ ) {
      // self.thumbnail = results[i];  Not any more
      this.thumbnail = results[i]
      return resolve(result[i]);
    }
  }, function(error) {
    return reject(error);
  });
}