overImgClassName = 'rollover';
overImgPostfix = '_on';
function rollOver() {
    var btns = $A($$('.' + overImgClassName));
    btns.each(function (node){
        node.imgsrc = node.src;
        node.imgsrc_over = node.src.replace('.gif', overImgPostfix+'.gif').replace('.jpg', overImgPostfix+'.jpg');
        node.onmouseover = function() { this.src = this.imgsrc_over; };
        node.onmouseout = function() { this.src = this.imgsrc; };
    });
}

window.addEvent('domready', rollOver);

function debug(obj) {
    message = 'self:' + obj + "\n";
    for (i in obj) {
        message += i + ":" + obj[i] + "\n";
    }
    var debugArea = document.createElement('pre');
    debugArea.appendChild(document.createTextNode(message));
    document.getElementsByTagName('body')[0].appendChild(debugArea);
}