They are using Flash, but you could come real close to this in JavaScript using jQuery.
1) Hide the img element (put it in a DIV with a white background)
2) Set the source
$("#yourImageID").attr("src", "your/path/your_image.png");
3) fadeIn() the img element
If you want better performance, search for image caching techniques because those images might take a minute to load. You would then swap in the cached images instead of loading them from the server as above.
That should get you started. Feel free to post new questions when you have specifics/issues related to implementation.
Cheers.