Parallelize get_serving_url in GAE Python

Question

The Traces graph showed /images.GetUrlBase calls for my search function. I tried to make it call get_serving_url () in parallel but still the Traces graph shows the calls as sequential.
How do I make it parallel ?

My Attempt

class Gets 
    @staticmethod 
    @ndb.tasklet 
    def get_img_url_parallel ( image_key ): 
        url     = yield images.get_serving_url_async (image_key) 
        raise ndb.Return ( url ) 


@toplevel 
def search (..): 
    :: 
    for record in record_list: 
        :: 
        for photo in record.photo_list: 
            if (photo != ""): 
                yielded_photo   = yield Gets.get_img_url_parallel ( photo ) 
                photo_list.append ( yielded_photo  ) 
::

I am new to yield, tasklet.