Class Post(model.Models):
#...
photo = models.ImageField(upload_to="photo_album")
original_photo = models.ImageField(upload_to="photo_album/originals", null=True)
thumbnail = models.ImageField(upload_to="photo_album/thumbnails", null=True)
def save(self):
self.thumbnail.file = self.photo.file
self.original_photo.file = self.photo.file
super(Post, self).save()