/* Reset */
* { margin: 0; padding: 0; box-sizing: border-box; }

html, body {
    height: 100%;
    width: 100%;
    font-family: Arial, sans-serif;
}

/* Default: mobile background */
body {
    background-image: url("background.jpeg"); /* <-- YOU PUT YOUR MOBILE IMAGE */
    background-size: cover;
    background-position: center;
    background-repeat: no-repeat;
}

/* Desktop-landscape class (activated by JS) */
.desktop-landscape {
    background-image: url("DESKTOP-IMAGE.jpg"); /* <-- YOU PUT DESKTOP IMAGE */
}