I think you'll need to place them in an image folder and change the css source to point to their url which shouldn't be a big issue. I've quoted the relevant section below (I hope)

Nice find by the way Rob

section.module.parallax-1 {
background-image: url("../img/demo/_small/1.jpg");
}
section.module.parallax-2 {
background-image: url("../img/demo/_small/2.jpg");
}
section.module.parallax-3 {
background-image: url("../img/demo/_small/3.jpg");
}