body {
        margin: unset;
    }
    
    .imagen {
        background: url('../img/back.jpg') no-repeat center center fixed;
        -webkit-background-size: cover;
        -moz-background-size: cover;
        -o-background-size: cover;
        background-size: cover;
        background-size: cover;
        background-repeat: no-repeat;
        background-position: top;
        height: 100vh;
    }
    
    @media only screen and (max-width: 600px) {
        .imagen {
            background: url('../img/back_m.jpg') no-repeat center center fixed;
            -webkit-background-size: cover;
            -moz-background-size: cover;
            -o-background-size: cover;
            background-size: cover;
            background-size: cover;
            background-repeat: no-repeat;
            background-position: top;
            height: 100vh;
        }
    }