Colour Variation Images - hoping to have them change when colour is slected

Heya,
I have some colours for my hats and you can see the colour variations photo if you choose "Choose Options" via the popup here - http://www.flaminmongrels.com/store/products/

But if you click through to the individual product page here - http://www.flaminmongrels.com/store/products/flamin-mongrels-trucker-cap/
the photo does not change when you choose a different colour.

Thanks for you help...
Bretto