var randnum = Math.random();
var inum = 6;// Change this number to the number of images you are using.
var rand1 = Math.round(randnum * (inum-1)) + 1;
images = new Array
images[1] = "images/front/header_bg_1.jpg"
images[2] = "images/front/header_bg_2.jpg"
images[3] = "images/front/header_bg_3.jpg"
images[4] = "images/front/header_bg_4.jpg"
images[5] = "images/front/header_bg_5.jpg"
images[6] = "images/front/header_bg_6.jpg"

// Ensure you have an array item for every image you are using.
var image = images[rand1]