For easy things like that, especially if you're unfamiliar with JavaScript, you can try some very easy frameworks like jQuery.
jQuery Tutorials here (download links at top):
http://docs.jquery.com/Tutorials
I didn't look (but you can) and they're most likely sending out an AJAX request to download a new image, layering 2 images over each other in the same place and fading the new image in on top of the old image.
jQuery has a simple AJAX syntax:
http://api.jquery.com/jQuery.ajax/
And to fade something in, a simple effect:
http://api.jquery.com/fadeIn/
Look at the sample code provided on those links. You can basically copy and paste that effect easily and jQuery is very consistent between all browsers, OSs and versions.
You're welcome.
