var imagenumber = 6;
var randomnumber = Math.random() ;
var rand1 = Math.round( (imagenumber-1) * randomnumber) + 1 ;

images = new Array
images[1] = "img/1.jpg";
images[2] = "img/2.jpg";
images[3] = "img/3.jpg";
images[4] = "img/4.jpg";
images[5] = "img/5.jpg";
images[6] = "img/6.jpg";
var image = images[rand1];