The website you linked doesn't changes the image's width but it actually cuts it off. For that it needs to be set as a background-image.
For more info about background-image
look it at http://www.w3schools.com/cssref/pr_background-image.asp
Usage:
#divID {
background-image:url(image_url);
-webkit-background-size: cover;
-moz-background-size: cover;
-o-background-size: cover;
background-size: cover;
}