function replaceImages() {
	var images = document.getElementsByTagName("img");
	for(var i=0; i < images.length; i++) {
		if(images[i].getAttribute("src").match("/bt..a/")); {
			images[i].onmouseover = function() {
				this.setAttribute("src", this.getAttribute("src").replace("a.", "b."));
			}
			images[i].onmouseout = function() {
				this.setAttribute("src", this.getAttribute("src").replace("b.", "a."));
			}
		}
	}
}

function preloadImages() {
	for(var i=1; i <= 6; i++) {
		loadImage = new Image();
		loadImage.src = "bt0" + i + "b.png";
	}
}

function callFunctions() {
	preloadImages();
	replaceImages();
}

if(window.addEventListener) {
	window.addEventListener("load", callFunctions, false);
}  
else if(window.attachEvent) {
	window.attachEvent("onload", callFunctions);
}