var mess = new Array
('<img src="img/bannerverticaladsh.jpg" alt="Sponsored by streetheritage.com" width="120" height="500">',
'<img src="img/bannerverticaladsh2.jpg" alt="Sponsored by streetheritage.com" width="120" height="500">'
);
var max = mess.length;
var num = Math.floor((Math.random() * max));
document.writeln(mess[num]);