img {
  max-width: 100%;
}
var randomList = new Array(
  "https://bties.co.jp/img/bg02.jpg",
  "https://bties.co.jp/img/bg03.jpg",
  "https://bties.co.jp/img/bg04.jpg",
  "https://bties.co.jp/img/bg05.jpg" );
var num = Math.floor(Math.random() * randomList.length);
var printHtml = '<img src=' + randomList[num] + ' alt="ランダム画像">';
document.write(printHtml);

External CSS

This Pen doesn't use any external CSS resources.

External JavaScript

This Pen doesn't use any external JavaScript resources.