Crossfade Images on Body

Building a WordPress theme and trying to crossfade multiple background images on the body tag and not on a div! I have been trying to use the tutorial here: http://ift.tt/1o25R5v | Demo 3 with no success

Is it possibe to do without javascript and jquery and just use css only! Currently my Body css looks like this....

body {
    font-family: 'Arial', courier, sans-serif;
    background:url('ASSETS/bg2.jpg')no-repeat top left;
    background-size:cover;
    background-attachment:fixed
}

I do have another image to crossfade in my ASSETS folder called bg.jpg.