how do they do that?

[Charles A. Landemaine]

View the page source. It uses a combination of: Prototype.js,
scriptaculous.js and lightbox.js. The effect may look cool the first
couple of times you see it, but it very quickly becomes a boring waste
of time while you wait for the image to display.

Yeah, lightbox javascript is super famous on the Internet. I also prefer
a regular popup window.