Add the ability to turn off image pulling.

Other disadvantages:
* Clients can't introspect default behavior
* Creates a subtle API version dependency: Changing default behavior is a breaking API change. If we were to ever change such defaults, we'd need to factor out the code into a version-specific location. Furthermore, if we were to allow objects to survive API version changes (which we probably will eventually), we'd want to record the original default in the object.