var mess = new Array('<img src="images/class_trap.jpg" width="291" height="194" border="0">','<img src="images/bb_lect2.jpg" width="291" height="194"border="0">','<img src="images/students_trap2.jpg"  width="291" height="194" border="0">','<img src="images/SMclass.jpg"  width="291" height="194" border="0">');
var max = mess.length;
var num = Math.floor((Math.random() * max));
document.writeln(mess[num]);