function mainimage(){
  img = new Array();
  img[0] = "/assets/top_img/mainimage_1.jpg";
  img[1] = "/assets/top_img/mainimage_1.jpg";
  img[2] = "/assets/top_img/mainimage_2.jpg";
  img[3] = "/assets/top_img/mainimage_1.jpg";
  img[4] = "/assets/top_img/mainimage_1.jpg";

  n = Math.floor(Math.random()*img.length);
  document.write("<img src='"+img[n]+"' width='630' height='190' alt=''>");
}