images = new Array(4);

images[0] = "<style type='text/css'>body{ background: transparent url(http://www.amypink.com/images/backs/back1.jpg) fixed 50% 0;}</style>";

images[1] = "<style type='text/css'>body{ background: transparent url(http://www.amypink.com/images/backs/back2.jpg) fixed 50% 0;}</style>";

images[2] = "<style type='text/css'>body{ background: transparent url(http://www.amypink.com/images/backs/back3.jpg) fixed 50% 0;}</style>";

images[3] = "<style type='text/css'>body{ background: transparent url(http://www.amypink.com/images/backs/back4.jpg) fixed 50% 0;}</style>";

index = Math.floor(Math.random() * images.length);

document.write(images[index]);