mouseover image swap with preloader unobtrusive

function imageSwap(id) {
		var links = document.getElementById(id).getElementsByTagName("a");
		var imgLoad = []
 
		for(var i = 0; i < links.length; i++) {
			attachBehavior(links[i], i);
		}
 
		function attachBehavior(obj, iter) {
 
			var img = obj.getElementsByTagName('img')[0];
			var imgSrc = img.getAttribute("src");
			var ext = imgSrc.match(/\.\S{3}$/);
			var overSrc = imgSrc.replace(ext, "-over" + ext);
 
                        // preLoad over states
			imgLoad[iter] = new Image();
			imgLoad[iter].src = overSrc
 
                        // use event listeners if appropriate
			obj.onmouseover = function(){
				img.setAttribute("src", overSrc);
			}
			obj.onmouseout = function(){
				img.setAttribute("src", imgSrc);
			}
		}
	}
 
	imageSwap("footer-links");
 
        // takes an image in html page of the form about_us_link.gif and swaps it with an image                  
        // about_us_link.gif and replaces it with image named about_us_link-over.gif
        // Create a new image for mouseover named the same as the original image
        // with "-over" appended to the end but before the extension for example