I'm still new to asp .net, but check out some tutorials on asp Update Panels and AJAX.
Basically, the attribute selector dropdown menu lives in an Update Panel, and when the value of the dropdown changes, a Label control within the SAME Update Panel gets populated with the attribute information. All of this takes place asynchronously... basically meaning the entire page doesn't reload, just the information within the Update Panel.
You need to do the same thing with the product image control (defaultImage). You will need to make sure that the Image control is located within an Update Panel. Then you would use the "Triggers" feature of Update Panels to allow the Attributes Panel to talk to the Image Panel. If it were me, I would append the SKU suffix of the currently selected attribute onto the end of the image name, and load that image.
Your image structure would then need to be as follows:
Web/repository/product/ :
defaultImage.jpg
defaultImage_skuColorValue1.jpg
defaultImage_skuColorValue2.jpg
... etc.
Hope that helps...
~Ryan