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

#intro {
    background: url("http://lavarde.free.fr/plage/qualite/plage2.jpg")no-repeat center center fixed;
    -webkit-background-size: cover;
    -moz-background-size: cover;
    -o-background-size: cover;
    background-size: cover;
}