#hero {
    background-image: url('img/hero-bg-desktop.png');
    background-repeat: no-repeat;
    background-size: cover;
}
#footer {
    background-image: url('img/footer-bg-desktop.png');
    background-repeat: no-repeat;
    background-size: cover;
}

@media (max-width: 639px) {
    #hero {
        background-image: url('img/hero-bg-mobile.png');
        background-repeat: no-repeat;
        background-size: cover;
    }
    #footer {
        background-image: url('img/footer-bg-mobile.png');
        background-repeat: no-repeat;
        background-size: cover;
    }
}