arImages=new Array(
 
  "images/class1.jpg",
  "images/class2.jpg",
  "images/class3.jpg",
  "images/class4.jpg",
  "images/class5.jpg",
  "images/class6.jpg",  
  "images/class7.jpg",
  "images/class8.jpg",
  "images/class9.jpg"
);

rand=Math.floor(arImages.length*Math.random());
document.write('<img src="'+arImages[rand]+'" width="177" height="130" alt="Testing">');