var imglist = new Array (
  '/img/img01.jpg',
  '/img/img02.jpg',
  '/img/img03.jpg',
  '/img/img04.jpg',
  '/img/img05.jpg',
  '/img/img06.jpg',
  '/img/img07.jpg',
  '/img/img08.jpg',
  '/img/img09.jpg',
  '/img/img10.jpg',
  '/img/img11.jpg',
  '/img/img12.jpg',
  '/img/img13.jpg',
  '/img/img14.jpg',
  '/img/img15.jpg',
  '/img/img16.jpg',
  '/img/img17.jpg'
);

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

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