jQuery.preloadImages = function(images)
{
  for(var i = 0; i<images.length; i++)
  {
    jQuery("<img>").attr("src", images[i]);
  }
}

$(function() {
  var bkg_imgs = ["/public/images/banner/b1.jpg",
                  "/public/images/banner/b2.jpg",                  
                  "/public/images/banner/b3.jpg"];
  $.preloadImages(bkg_imgs);
  var randomnummber = Math.floor(Math.random()*3 ),
      url =  bkg_imgs[randomnummber];
    $("#mgbanner img").attr("src", url);          
});