LedZeppelin.com resizing for a browsers

I don’t know which catagory to put this in…sorry feel free to move it if need be. I used to visit regularly here at the buzz. =o) Its been a while. ANYWAY…How do they do this?? It resizes when you resize the browser, its great! I’ve also seenit used on photos on purgedi.com….HOW HOW HOW! I think this is truly a designers dream. Or is it a buggy pain…fill me in please I want all the details!!

Thanks!
Kelly Halldorson