stretching background

[grabenair]

How do you stretch a background img to fit the whole browser window. I have googled this and can only find old stuff. What my client, my nephew, is looking for is something like they did here http://www.ryandungey.com/photos/category/race I tried to use firebug to figger out how they did it. But can not get it. I figger it is something simple I am missing, it useually is.

[virtual]

http://css-tricks.com/perfect-full-page-background-image/