if(navigator.appVersion.charAt(0) >= "4"){
  img = new Array();
  img[0] = new Image; img[0].src = "img/botton2.gif";
  img[1] = new Image; img[1].src = "img/bottona2.gif";
}
function chg_img(a,b){  //aは下のnameに対応し、bは上の配列に対応
  if(navigator.appVersion.charAt(0) >= "3"){
    document.images["img" +a].src=img[b].src;
  }
}