.multimedia-hero-slider li:nth-of-type(2) {
    background-image: url(../images/chairs-bg-mobile.png);
}

@media only screen and (min-width: 768px) {

    .multimedia-hero-slider li:nth-of-type(2) {
        background-image: url(../images/chairs-bg.jpg);
        background-position-y: -200px;
    }

}