django imagefield

class Post(models.Model):
    #...
    photo = models.ImageField(upload_to="photo_album")
    thumbnail = models.ImageField(upload_to="photo_album/thumbnails", null=True, blank=True)

    def save(self):
        thumb = Image.open(self.photo.file)
        thumb_io = StringIO.StringIO()
        thumb.save(thumb_io,format = thumb.format,)
        self.thumbnail.save(self.photo.name, ContentFile(thumb_io.getvalue()), save=False)
        super(Post, self).save()