Position absolute, fixed height, centered header image

gistfile1.css

/*
 * 1. Position if underneath a fixed navigation.
 * 2. Set to image height.
 */
 .header-image {
	position: absolute;
	overflow: hidden;
	top: px /* 1 */
	left: 0;
	width: 100%;
	height: px; /* 2 */	
}

/*
 * 1. Set to half image width (-ve).
 */
.header-image img {
	position: absolute;
	top: 0;
	left: 50%;
	margin-left: -px /* 1 */
}