Fetch image data from the Xeoma web server

            Arguments:
                image_name: the name of the image to fetch (i.e. image01)
                username: the username to directly access this image
                password: the password to directly access this image

async def async_fetch_image_data(self, image_name, username, password):
        """
            Fetch image data from the Xeoma web server

            Arguments:
                image_name: the name of the image to fetch (i.e. image01)
                username: the username to directly access this image
                password: the password to directly access this image
        """
        params = {}
        cookies = self.get_session_cookie()
        if username is not None and password is not None:
            params['user'] = self.encode_user(username, password)
        else:
            params['user'] = ''
        async with aiohttp.ClientSession(cookies=cookies) as session:
            resp = await session.get(
                '{}/{}.jpg'.format(self._base_url, image_name),
                params=params
            )
            if resp.headers['Content-Type'] == 'image/jpeg':
                data = await resp.read()
            else:
                data = None
        return data