Fluid Header Image like 2013 for 2011

Hi

As the title suggests I am looking to implement the fluid nature of the twenty thirteen header image on my existing twenty eleven child theme. The feature I like is that as the browser size is reduced, the image isn’t resized/scaled as such, just slides across. Can anyone tell me how I can achieve this?

I had a look at part of the code of 2013 and it appears to have this, but I can seem to get it working on my theme

.site-header {
	position: relative;
}

.site-header hgroup {
	-webkit-box-sizing: border-box;
	-moz-box-sizing:    border-box;
	-ms-box-sizing:     border-box;
	box-sizing:         border-box;
	margin: 0 auto;
	max-width: 1040px;
	min-height: 230px;
	width: 100%;
}