section.top {
    background: url(/img/fondo_inicio_m.jpg) center center no-repeat transparent;
    background-size: cover;
}
section.bottom {
    background: url(/img/fondo_cita_m.jpg) center center no-repeat transparent;
    background-size: cover;
}

@media screen and (min-width: 768px) and (max-width: 959px) {
    section.top {
        background: url(/img/fondo_inicio_t.jpg) center center no-repeat transparent;
        background-size: cover;
    }

    section.bottom {
        background: url(/img/fondo_cita_t.jpg) center center no-repeat transparent;
        background-size: cover;
    }
}

@media screen and (min-width: 960px) {
    section.top {
        background: url(/img/fondo_inicio.jpg) center center no-repeat transparent;
        background-size: cover;
    }

    section.bottom {
        background: url(/img/fondo_cita.jpg) center center no-repeat transparent;
        background-size: cover;
    }
}