var imglist = new Array (
  '/img/img_main000.jpg',
  '/img/img_main001.jpg',
  '/img/img_main002.jpg',
  '/img/img_main003.jpg',
  '/img/img_main004.jpg',
  '/img/img_main005.jpg',
  '/img/img_main006.jpg',
  '/img/img_main007.jpg',
  '/img/img_main008.jpg',
  '/img/img_main009.jpg',
  '/img/img_main010.jpg',
  '/img/img_main011.jpg',
  '/img/img_main012.jpg'
);

function fnRandomImage() {
  var selectnum = Math.floor(Math.random() * imglist.length);
  document.images['topimg'].src = imglist[selectnum];
}

window.onload = function() {
  fnRandomImage();
}