var image = new Array();
for(i = 0; i < 5; i++){
  image[i] = new Image();
  }
  image[0].src = "http://www.yukai-hokkaido.jp/special/images/honshi/hokkaido_hyoushi.jpg";
  image[1].src = "http://www.yukai-life.jp/special/images/honshi/shuto_hyoushi.jpg";
  image[2].src = "http://www.yukai-tokai.jp/special/images/honshi/tokai_hyoushi.jpg";
  image[3].src = "http://www.yukai-kansai.jp/special/images/honshi/kansai_hyoushi.jpg";
  image[4].src = "http://www.yukai-kyushu.jp/special/images/honshi/kyushu_hyoushi.jpg";
  
function chgImage(mediaIMG,n){
    document.images[mediaIMG].src = image[n].src;
}