'given image index, find out full path
:param index: index of a specific image
:return: full path of this image'

def image_path_from_index(self, index):

    image_file = os.path.join(self.data_path, 'JPEGImages', (index + '.jpg'))
    assert os.path.exists(image_file), 'Path    does    not    exist:    {}'.format(image_file)
    return image_file
