code: function (object, images) {
              if (object) {
                  var skinImageProperties = this.skinImageProperties;
                  for (var i = 0, l = skinImageProperties.length; i < l; i++) {
                      var propName = skinImageProperties[i];
                      var value = object[propName];
                      if (value) {
                          images[value] = 1;
                      }
                  }
              }
          }
docs: Extract skin images from an object from the skin and add them to a map.
  @param {Object} object Object from the skin. It can be either a skin class, a state, a frame or frame state
  object. Any property of this object whose name is inside this.skinImageProperties is supposed to contain the
  URL of an image.
  @param {Object} images Map to which the image urls will be added. Each key in the map is an object URL.