/* image preloading */
function preload(imgName, imgSrc) {
    if (loadFlag) {
        eval(imgName + ' = new Image()');
        eval(imgName + '.src = "' + imgSrc + '"');
        return imgName;
    }
}

/* turn image on */
function mson(imgName) {
    document[imgName].src = eval(imgName + "_On.src");
}

/* turn image off */
function msoff(imgName) {
    document[imgName].src = eval(imgName + "_Off.src");
}

/* switch an image */
function changeImg(imgName, newImg) {
    document[imgName].src = eval(newImg + ".src");
}