var randnum = Math.random();
var inum = 12;

// Change this number to the number of images you are using.
var rand1 = Math.round(randnum * (inum-1)) + 1;
images = new Array
images[1] = "backgrounds/01.jpg"
images[2] = "backgrounds/02.jpg"
images[3] = "backgrounds/03.jpg"
images[4] = "backgrounds/04.jpg"
images[5] = "backgrounds/05.jpg"
images[6] = "backgrounds/06.jpg"
images[7] = "backgrounds/07.jpg"
images[8] = "backgrounds/08.jpg"
images[9] = "backgrounds/09.jpg"
images[10] = "backgrounds/10.jpg"
images[11] = "backgrounds/11.jpg"
images[12] = "backgrounds/12.jpg"

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