$(document).ready(function() { 

var img = new Array() 
img[0] = "IMG/bg/1.jpg", 
img[1] = "IMG/bg/2.jpg",
img[2] = "IMG/bg/3.jpg",
img[3] = "IMG/bg/4.jpg",
img[4] = "IMG/bg/5.jpg",
img[5] = "IMG/bg/6.jpg",
img[6] = "IMG/bg/7.jpg",
img[7] = "IMG/bg/8.jpg",
img[8] = "IMG/bg/9.jpg",
img[9] = "IMG/bg/10.jpg",
img[10] = "IMG/bg/11.jpg",
img[11] = "IMG/bg/12.jpg",
img[12] = "IMG/bg/13.jpg",
img[13] = "IMG/bg/14.jpg"
img[14] = "IMG/bg/15.jpg",
img[15] = "IMG/bg/16.jpg",
img[16] = "IMG/bg/17.jpg",
img[17] = "IMG/bg/18.jpg",
img[18] = "IMG/bg/19.jpg",
img[19] = "IMG/bg/20.jpg",
img[20] = "IMG/bg/21.jpg",
img[21] = "IMG/bg/22.jpg";

var rnd = Math.round(Math.random()* (img.length - 1)); 

$('#arriereplan').css('background', 'url('+img[rnd]+') repeat fixed top center'); 

$('#arriereplan').css('width', '100%'); 
$('#arriereplan').css('height', '100%'); 

$('#arriereplan').css('left', '0px'); 
$('#arriereplan').css('top', '0px'); 

$('#arriereplan').fadeIn(1000);

});