function rollover() {
    var images = document.getElementsByTagName("img");
    for (var i = 0; i < images.length; i++) {
        images[i].onmouseover = function() { this.src = this.id + "_over.png"; }
        images[i].onmouseout = function() { this.src = this.id + "_default.png"; }
    }
}
window.onload = rollover;