html,
body,
header,
#intro {
    height: 100%;
}

#intro {
    background: url("https://i.ibb.co/J24Zy0f/12939-escuela-familias.jpg")no-repeat center center fixed; 
    -webkit-background-size: cover;
    -moz-background-size: cover;
    -o-background-size: cover;
    background-size: cover;
}