'Return the ``Application`` image name and tag.
:returns: ``unicode`` representing the image name and tag or ``None``
if the image is unknown (``None`` or not a ``DockerImage``).'

def convert_image(self):

    if isinstance(self._application.image, DockerImage):
        return u':'.join([self._application.image.repository, self._application.image.tag])
    return None
