Add the ability to turn off image pulling.

along the way we lost the ToUpper() or ToLower() to make these case-insensitive.

As a different question: This pattern (IsFoo() functions which encode the if-not-specified behavior) vs defaulting values in validation?  This pattern has a distinct advantage of working in tests where validation has not run.  It has the distinct disadvantage that not all fields are so self-contained - some set their default based on another field's value.

I'd like to be consistent as much as possible. I'll file an issue.  No urgency.