Resurrecting this thread to thank Lyle specifically, and the other posters generally.
I would never have figured this out without his simple reply. My situation: I wanted the default option image to appear when the product page is first loaded. I guess my quibble is with the word "required". The products I'm dealing with can be ordered in many colors, and the customer must choose a color - it is 'required'! But checking the 'required' check box caused the phrase 'Please Select' to appear as the default in the select list drop-down. That meant that none of the option images would appear until you chose one.
This seemed counterintuitive.
Un-selecting 'Required' in the attributes settings makes the option images work the way I want. ... And looking at it now, I know that if a customer doesn't want the product in the default color, they will just switch (duh!).
This is a cool feature, so thanks again all around.