function btnMouseOver(imgName, img) {
    var imgName = "Images/btn_" + imgName + "_on.gif";
    var imgElm = document.getElementById(img);
    imgElm.src = imgName;
}

function btnMouseOut(imgName, img) {
    var imgName = "Images/btn_" + imgName + ".gif";
    var imgElm = document.getElementById(img);
    imgElm.src = imgName;
}