code: func (ts *Store) GetImageHash(id string) (string, error) {
  	treepath := ts.GetPath(id)
  
  	imgHash, err := ioutil.ReadFile(filepath.Join(treepath, imagefilename))
  	if err != nil {
  		return "", errwrap.Wrap(errors.New("cannot read image file"), err)
  	}
  
  	return string(imgHash), nil
  }
docs: // GetImageHash returns the hash of the image that uses the tree store
  // identified by id.