// JavaScript Document

var classSet = new Array
var xx = 0;

classSet[xx++] = 'bg1';
classSet[xx++] = 'bg2';
classSet[xx++] = 'bg3';


function replaceIMG(idNode) {
  if(!document.getElementById(idNode)) return;
  
  var elementNode = document.getElementById(idNode);
  var ii = Math.floor(Math.random()*classSet.length);
  
  elementNode.className = classSet[ii];
}