function PreLoadImg() {
  ImgFiles = PreLoadImg.arguments;
  document.PreLoadImgs = new Array();
  for (i=0; i<ImgFiles.length; i++){
    document.PreLoadImgs[i]     = new Image();
    document.PreLoadImgs[i].src = ImgFiles[i];
  }
}
function SwapImg(DocImgName,PreLoadImgNum) {
  // remember original ImgName & ImgSrc
  document.SwapDocImgName = DocImgName;
  document.SwapDocImgSrc  = document.images[DocImgName].src;
  // swap in new src
  document.images[DocImgName].src = document.PreLoadImgs[PreLoadImgNum].src;
}
function SwapImgRestore() {
  document.images[document.SwapDocImgName].src = document.SwapDocImgSrc;
}