window.onload = rolloverInit;

function rolloverInit() {
    for (var i = 0; i < document.images.length; i++) {
        if (document.images[i].parentNode.tagName == "A") {
            setupRollover(document.images[i]);
        }
    }
}

function setupRollover(thisImage) {
    thisImage.outImage = new Image();
    thisImage.outImage.src = thisImage.src;
    thisImage.onmouseout = rollOut;

    thisImage.overImage = new Image();
    thisImage.overImage.src = "images/" + thisImage.id + "_on.png";
    thisImage.onmouseover = rollOver;
}

function rollOver() {
    this.src = this.overImage.src;
}

function rollOut() {
    this.src = this.outImage.src;
}