function ch_bgimg(id, num) {
  if (navigator.appVersion.charAt(0) >= 3) {
    document.getElementById(id).style.background = "url(" + img[num].src + ") left top repeat-x";
  }
}

function ch_img(id, num) {
  if (navigator.appVersion.charAt(0) >= 3) {
    document.getElementById(id).src = img[num].src;
  }
}