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.jpg";
            thisImage.onmouseover = rollOver;

            thisImage.clickImage = new Image();
            thisImage.clickImage.src = "images/" + thisImage.id + "_click.jpg";
            thisImage.onmousedown = rollClick;       
}

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

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

function rollClick() {
            this.src = this.clickImage.src;
}