.topnav { background:rgba(0,0,0,0.5) url(../img/pusta88_cny.gif) repeat-x;background-size:15%; }

@media only screen and (max-width: 576px) {
    .topnav { background:rgba(0,0,0,0.5) url(../img/pusta88_cny.gif) repeat-x;background-size:30%; }
}