Estoy tratando de descargar mi archivo de URLField y descargarlo a ImageField. lo que me sale es error

 'utf-8' codec can't decode byte 0x89 in position 0: invalid start byte Unicode error hint The string that could not be encoded/decoded was: �PNG

modelo:

 image = models.ImageField(upload_to='posts/', blank=True) image_url = models.URLField(null=True, blank=True) def save(self, *args, **kwargs): if self.image_url: result = urllib.request.urlretrieve(self.image_url) self.image_url = '' self.image.save( os.path.basename(self.image_url), File(open(result[0])) ) super(Post, self).save()