Comment 1 for bug 1243219

There is a css option

 background-size: cover;

that scales the background image to be as large as possible so that the background area is completely covered by the background image. Some parts of the background image may not be in view within the background positioning area. http://www.w3schools.com/cssref/css3_pr_background-size.asp

However the background-size property is only supported in IE9+, Firefox 4+, Opera, Chrome, and Safari 5+ and not IE 8.

but saying that I notice that IE 11 was launched last week so we could probably drop support for IE8 so then we could use this css option :)