This is something that I have seen designers do by using the extra product fields, resize image option in ShopSite and a custom more information page template. An example of a website doing this is:
http://www.mogulinterior.com/product2284.html
What they have done is set up the JavaScript to that if the extra fields are used, then it will add the image file name from that extra field into the JavaScript in the HEAD on the page to preload that image.
For the thumbnail and the larger image, they are using the resize image option within ShopSite. They have put all of their images into the same folder (such as, all in the media folder) then when they create resized images, the URLs will always start with:
media/ss_size1/
media/ss_size2/
They can then take that same image file name that was entered in the extra field, and paste it into the Javascript on the main body of the page to create the thumbnail image, and the hover over image.
When setting up this way, they can have a max of 10 thumbnail images (there are ten extra fields).
-Lauren