.wrapper .container img { width: 100%; }
.wrapper .container img#btnYellowId { display: none; } 
@media (max-width: 720px) {
    nav { display: none; }
    footer { display: none; }
    .row { margin: 0px; }
    .wrapper .container { padding: 0px; }
    .wrapper .container img#btnYellowId { display: block; }
}