class Post(models.Model):

    #...
    photo = models.ImageField(upload_to="photo_album")
    medium_photo = models.ImageField(upload_to="photo_album/medium", null=True)
    thumbnail = models.ImageField(upload_to="photo_album/thumbnails", null=True)

def save(self):

        self.thumbnail.file = self.photo.file
        self.medium_photo.file = self.photo.file
        super(Post, self).save()