@media (max-width: 767px) {
    .wp-caption.alignleft, 
    .wp-caption.alignright,
    .alignleft,
    .alignright {
        float: none !important;
        display: block !important;
        margin-left: auto !important;
        margin-right: auto !important;
        max-width: 100% !important;
        text-align: center;
    }
}