Return png string of image.

def png(self):
        """Return png string of image."""
        use_plugin('freeimage')
        with TemporaryFilePath(suffix='.png') as tmp:
            safe_range_im = 255 * normalise(self)
            imsave(tmp.fpath, safe_range_im.astype(np.uint8))
            with open(tmp.fpath, 'rb') as fh:
                return fh.read()