JSFiddle - React, Tailwind, and code Playground
HTML
<div id="mostrarADS"></div>
JavaScript
Imagen = new Array('http://www.rsc.org/images/011107_Leaderboard_728x90px_2_tcm18-201691.gif', 'http://comparegithosting.com/_media/ads728.png','http://playnstuff.com/wp-content/themes/StartWPA/images/728.png');
//Obtenemos un valor al azar
var X = Math.floor((Math.random() * Imagen.length));
document.getElementById("mostrarADS").innerHTML = '<a href="http://www.google.com.ar" target="_blank"><img src="'+ Imagen[X] +'" border="0"></a>';