$(document).ready(function() {
  var randomClass = ['img1','img2','img3','img4','img5','img6','img7','img8','img9','img10','img11','img12','img13','img14','img15','img16'];
  var rndNum = Math.floor(Math.random() * randomClass.length);
  $("div#sidebarimg").addClass(randomClass[rndNum]);
});