It's a pretty standard free Java script that has about 4 dozen variations floating around on the net. Do a google search of "[link=http://www.google.com/search?sourceid=navclient&aq=t&ie=UTF-8&rlz=1T4SUNA_enUS225US226&q=thumbnail+viewer+java script]thumbnail viewer javascript[/link]" and you will have a list of different scripts you can use to get the same effect. Choose the one suits your needs the best.