def create_thumb(sender, instance, signal, *args, **kwargs):
    if path.exists(instance.thumb_file):
	return

    image = PIL.open(instance.image_file)
    image.thumbnail((256, 192), PIL.ANTIALIAS)
    image.save(instance.thumb_file)

def delete_thumb(sender, instance, signal, *args, **kwargs):
    if path.exists(instance.thumb_file):
        remove(instance.thumb_file)
    print 'delete', instance.pk, image

dispatcher.connect(create_thumb, signal=models.signals.post_save, sender=Image)
dispatcher.connect(delete_thumb, signal=models.signals.post_delete, sender=Image)