public static Uri addPlaceholder(Bitmap placeholder){
  Uri uri=generateUniquePlaceholderUri();
  replacePlaceholder(uri,placeholder);
  return uri;
}

public static Uri addPlaceholder ( Bitmap placeholder ) { Uri uri = generateUniquePlaceholderUri ( ) ; replacePlaceholder ( uri , placeholder ) ; return uri ; }
add a placeholder for a new image that does not exist yet .
