if (window.addEventListener) {
	window.addEventListener("load", initHeader, false) //invoke function
} else if (window.attachEvent) {
	window.attachEvent("onload", initHeader) //invoke function 
}

var bookOrig; var artOrig; var uspOrig;
var book; var art; var usp;

	function initHeader() {
		// preloading
		book = document.getElementById('header_book');
		art = document.getElementById('header_art');
		usp = document.getElementById('header_usp');
		
		// preloading
		var img1 = new Image(); var img2 = new Image(); var img3 = new Image();
		img1.src = book.src.toString().replace('_off.jpg','_over.jpg');
		img2.src = art.src.toString().replace('_off.jpg','_over.jpg');
		img3.src = usp.src.toString().replace('_off.jpg','_over.jpg');
		
		bookOrig = book.src;
		artOrig = art.src;
		uspOrig = usp.src;
	}
	
	function headerOver(qual) {
		if (book && art && usp) {
			if (qual == 'book') {
				book.src = book.src.toString().replace('_off.jpg','_over.jpg');
				art.src = artOrig;
				usp.src = uspOrig;
			} else if (qual == 'art') {
				book.src = bookOrig;
				art.src = art.src.toString().replace('_off.jpg','_over.jpg');
				usp.src = uspOrig;
			} else if (qual == 'usp') {
				book.src = bookOrig;
				art.src = artOrig;
				usp.src = usp.src.toString().replace('_off.jpg','_over.jpg');
			} else {
				book.src = bookOrig;
				art.src = artOrig;
				usp.src = uspOrig;
			}
		}
	}