if (document.images) {
   img1 = new Image(); img1.src = "imgs/bt-home.jpg";
   img2 = new Image(); img2.src = "imgs/bt-guide.jpg";
   img3 = new Image(); img3.src = "imgs/bt-yoyaku.jpg";
   img4 = new Image(); img4.src = "imgs/bt-kushitsu.jpg";
   img5 = new Image(); img5.src = "imgs/bt-access.jpg";
   img6 = new Image(); img6.src = "imgs/bt-tinfo.jpg";
}
function setImage(img, image) {
   if (img) {
      img.src = image;
   }
}