How are they doing this

Heu guys,
could you please check the following link:

http://designarchives.aiga.org/

please check the part where the images can be enlarged.
Can some one tell me how they doing the preloading?
is there a blurred up low-res image superimposed over and masked away just before the hi-res image is previewed?
probably its something very simple.

please post your comments.

Thanx,
Vulpine