code: public Observable<String> deleteImageAsync(String listId, String imageId) {
          return deleteImageWithServiceResponseAsync(listId, imageId).map(new Func1<ServiceResponse<String>, String>() {
              @Override
              public String call(ServiceResponse<String> response) {
                  return response.body();
              }
          });
      }
docs: Deletes an image from the list with list Id and image Id passed.
  
  @param listId List Id of the image list.
  @param imageId Id of the image.
  @throws IllegalArgumentException thrown if parameters fail the validation
  @return the observable to the String object