Image to always fill div

I'm trying to create a slideshow with full screen images that fill the viewport (in my case probably a div) like the ones on this site:

http://tympanus.net/Tutorials/CSS3FullscreenSlideshow/

The goal is for the image to ALWAYS fill the the div, regardless of its size... cropping the sides if the div is more narrow than the image, and cropping the top and bottom is the div is shorter than the image. Of course the image needs to maintain it's aspect ration.

Thanks in advance.