.dx-gallery {
    position: absolute;

}

.gallery-item {
    height: 100%;
}

@media only screen 
and (orientation : portrait) {
    .dx-gallery-item {
        display: table;
    }
    .gallery-item {
        display: table-cell;
        vertical-align: middle;
    }
    .gallery-item > img {
        width: 100%;
    }
}

@media only screen 
and (orientation : landscape) {
    .gallery-item  {
        text-align: center;
    }
    .gallery-item > img {
        height: 100%;
    }
}