+1  A: 

Given a string Bistro, is it possible to retrieve its corresponding model of the same name in Django?

Yes.

model = ContentType.objects.get(model='Bistro').model_class()

http://docs.djangoproject.com/en/dev/ref/contrib/contenttypes/

celopes
A thought about the approach: maybe you should use the ContentType framework to have foreign-keys in the Image class, and have callables that compute `url` according to the values of the linked objects?
celopes
That's exactly what I'm looking for, let me give it a shot.
Thierry Lam