var imgDir = "images/";


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

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