@media (min-width: 640px) {
    .portfolio { column-gap: 10px; }
}

@media (min-width: 640px) and (max-width: 959.999px) {
    .portfolio { column-count: 2; }
}

@media (min-width: 960px) {
    .portfolio { column-count: 3; }
}

.portfolio > div {
    display: inline-block;
    width: 100%;
    padding: 10px 10px 20px 10px;
    box-sizing: border-box;
}

.portfolio picture {
    display: inline-block;
    padding: 10px;
    box-sizing: border-box;
    border: 1px solid #CCC;
}