var mess = new Array(
'<img src="../random/waterfall.jpg" alt="" />',
'<img src="../random/model.jpg" alt="" />',
'<img src="../random/flowers.jpg" alt="" />',
'<img src="../random/bee.jpg" alt="" />');
var max = mess.length;
var num = Math.floor((Math.random() * max));
document.writeln(mess[num]);