section {
    position: relative;
    width: 100%;
    height: 100vh;

    background-color: #000;
    background-image: url('background.gif');
    background-size: cover;
    background-repeat: no-repeat;
    background-position: center;
    background-attachment: fixed;
}

@media screen and (max-device-width: 480px) {
    section {
        background-size: 100% auto;
    }
}