.header {
    width: 60%;
    margin: 20px auto 20px auto;
    text-align: center;
    letter-spacing: 1.0em;
}

.ImageContainer {
    column-count: 3;
    grid-gap: 5px;
    width: 80%;
    margin: auto;
}

.block {
    display: inline-block;
    width: 100%;
}

    .block img {
        width: 100%;
    }

@media screen and (max-width: 720px) {

    .ImageContainer {
        column-count: 2;
        width: 80%;
    }
}

@media screen and (max-width: 480px) {

    .ImageContainer {
        column-count: 1;
        width: 80%;
    }
}