.page-treatment .main-visual__child {
    background: url(../img/mv.jpg) no-repeat 50%;
    background-size: cover;
}

@media only screen and (max-width:979px){
    .page-treatment .main-visual__child {
        background: url(../img/mv_sp.jpg) no-repeat 50%;
        background-size: cover;
    }
}