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(0);
    thisImage.overImage.src = "images/"+thisImage.id+"_over.jpg";
    thisImage.onmouseover = rollOver;
}

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

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

window.onload=rolloverInit();