var photos = new Array();

photos[0] = "advert1.jpg";
photos[1] = "advert2.jpg";
photos[2] = "advert3.jpg";
photos[3] = "advert4.jpg";
photos[4] = "advert5.jpg";
photos[5] = "advert6.jpg";
photos[6] = "advert7.jpg";

var whichSet = Math.round(Math.random()*(photos.length-1));