code: function handleImageLoad (imageUrl) {
    _backingStores.forEach(function (backingStore) {
      if (layerContainsImage(backingStore.layer, imageUrl)) {
        invalidateBackingStore(backingStore.id);
      }
    });
  }
docs: Invalidates the backing stores for layers which contain an image layer
  associated with the given imageUrl.
  
  @param {String} imageUrl