var randnum = Math.random();
var inum = 6;

var rand1 = Math.round(randnum * (inum-1)) + 1;
var images = new Array
images[1] = "images/back/1.jpg"
images[2] = "images/back/2.jpg"
images[3] = "images/back/3.jpg"
images[4] = "images/back/4.jpg"
images[5] = "images/back/5.jpg"
images[6] = "images/back/6.jpg"
images[7] = "images/back/7.jpg"
images[8] = "images/back/8.jpg"
images[9] = "images/back/9.jpg"
images[10] = "images/back/10.jpg"
images[11] = "images/back/11.jpg"
images[12] = "images/back/12.jpg"
images[13] = "images/back/13.jpg"
images[14] = "images/back/14.jpg"
images[15] = "images/back/15.jpg"
var image = images[rand1]