var preloaded = [];

   for (var i = 1; i <= 7; i++) {
    preloaded[i] = [loadImage(i + "-1.gif"), loadImage(i + "-1.gif")];   //设置图片的OnMouseOver和OnMouseOut的路径
   }

   function init() {
    if (mtDropDown.isSupported()) {
     mtDropDown.initialize();

    }
   }

   function loadImage(sFilename) {
    var img = new Image();
    img.src ="" + sFilename;
    return img;
   }

   function swapImage(imgName, sFilename) {
    document.images[imgName].src = sFilename;
   }