var randnum = Math.random();
var inum = 4;
var rand1 = Math.round(randnum * (inum-1)) + 1;

images = new Array
images[1] = "http://blog.se-nse.net/wp-content/themes/USEBv4/images/1.jpg"
images[2] = "http://blog.se-nse.net/wp-content/themes/USEBv4/images/2.jpg"
images[3] = "http://blog.se-nse.net/wp-content/themes/USEBv4/images/3.jpg"
images[4] = "http://blog.se-nse.net/wp-content/themes/USEBv4/images/4.jpg"

var image = images[rand1]

document.write('<style type="text/css">#menu {background: url(' + image + ') no-repeat;</style>')