function mxPreload(sDir) {
   this.sBase = sDir;
   this.iIndex = 0;
   this.arrImg = new Array();

   this.add = function (img) {
       this.arrImg[this.iIndex] = new Image;
       this.arrImg[this.iIndex].src = this.sBase + img;
       this.iIndex = this.iIndex + 1;
   }
}