//http://snipplr.com/view/3683/unobtrusive-mouseover-image-swap-with-preloader/
function imageSwap(id) {
    if (document.getElementById(id) != null) {
        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("navigationItems");
imageSwap("portfolioSelect");