Randomizing header images

Can someone please explain how the "randomize uploaded header" feature is supposed to work? I have header images enabled on all the site pages, and my expectation is that randomizing them would select a random image each time a page loads. That's not happening. Here's the site: http://www.uppervalleyfencingclub.com/wp/ I have 4 possible header images uploaded, and they are listed in the Previously uploaded section. But the same image keeps displaying, and it's the same on all pages. What's random about this feature? What am I missing? Thanks.