backgroundSize . js
background-size: cover; scales an image to cover entirely the background of a target element (the body of this page is styled this way).
background-size: contain; scales an image to make it as big as possible but always entirely visible (or contained) in the background of a target element (this box is styled this way).
background-size is available in IE9+, Firefox, Chrome, Safari and Opera.
Unfortunately, progressive enhancement doesn't work very well here: IE6-7-8 users would see a broken website.
jquery.backgroundSize.js is a 1.5k jQuery plugin that provides a fallback for those browsers:
Resize your browser to see how the background images are affected on this page.
NB: this page shows emulated
background-size to all browsers but it's possible to use CSS first, then load & use this library in conditionnal comments.