.gallery figure {
    /* tablet, mobile (default): 2 tiles */
    display: block;
    float: left;
    width: 50%
}
@media only screen and (min-width : 768px) {
    /* desktop: 4 tiles */
    .gallery figure {
        width: 25%
    }
}

.gallery figure figcaption {
    text-align: center
}

.gallery figure img {
    padding: 0 2px
}