var imgDir = "/jopt2005/images/";

function RollOn(imgName) { 
    if (document.images && document[imgName]) {
       document[imgName].src = imgDir + imgName + "_on.gif";
    }
}

function RollOff(imgName) { 
    if (document.images && document[imgName]) {
       document[imgName].src = imgDir + imgName + "_off.gif";
    }
}