$(document).ready(function(){
  
  var classes = new Array(
    "type1",
    "type2",
    "type3",
    "type4"
  );
  var classes_number = classes.length;
  var random_number = Math.round((classes_number-1)*Math.random());
  document.body.className = classes[random_number];

  var url = $("body #box").hide().css("backgroundImage");
  
  url = url.replace(/url\(/g, "");
  url = url.replace(/\)/g, "");
  url = url.replace(/\"/g, "");
  url = url.replace(/\'/g, "");
  
  var bg = new Image();
  bg.src = url;
  
  $(bg).css("display", "none");  
  $("body").append(bg);
  
  $(bg).ready(function(){
    $("body #box").show();
    $("#block").fadeIn(1500);
  });
});