img = new Array
img[0] = "img/img_podstrona_pociag.jpg";
img[1] = "img/img_podstrona_samochod.jpg";
img[2] = "img/img_podstrona_samolot.jpg";
img[3] = "img/img_podstrona_statek.jpg";

rnd = Math.floor(Math.random() * img.length);