window.onload = function() {
    if (document.getElementById("Navigation").getElementsByTagName("ul")) makeRoll();
}

function makeRoll() {
    var ssMenu = document.getElementById("Navigation").getElementsByTagName("ul")[0];
    for (var i = 0; i < ssMenu.getElementsByTagName("img").length; i++) {
        if (ssMenu.getElementsByTagName("img")[i].src.indexOf("_on.gif") == -1) {
            ssMenu.getElementsByTagName("img")[i].onmouseover = function() {
                this.src = this.src.replace(".gif", "_on.gif");
            }
            ssMenu.getElementsByTagName("img")[i].onmouseout = function() {
                this.src = this.src.replace("_on.gif", ".gif");
            }
        }
    }
}