main.css

body {
    margin: 0;
}

/* apply a natural box layout model to all elements, but allowing components to change */

html {
    box-sizing: border-box;
}

*,
*:before,
*:after {
    box-sizing: inherit;
}

.image-grid {
    width: 100%;
    max-width: 1310px;
    margin: 0 auto;
    overflow: hidden;
    padding: 10px 5px 0;
}

.image-cell {
    float: left;
    position: relative;
}

.image-basic {
    padding: 0 5px;
}

.basic-img {
    display: block;
    max-width: 100%;
    height: auto;
    margin: 0 auto;
}

.image-cell.is-collapsed .image-basic {
    cursor: pointer;
}

.image-cell.is-expanded .image-expand {
    max-height: 500px;
    margin-bottom: 10px;
}

.image-expand {
    position: relative;
    left: -5px;
    padding: 0 5px;
    box-sizing: content-box;
    overflow: hidden;
    background: #222;
    max-height: 0;
    transition: max-height .3s ease-in-out, margin-bottom .1s .2s;
}

.image-cell.is-collapsed .arrow-up {
    height: 10px;
    width: 100%;
}

.image-cell.is-expanded .arrow-up {
    border-bottom: 8px solid #222;
    border-left: 8px solid transparent;
    border-right: 8px solid transparent;
    height: 0;
    width: 0;
    margin: 2px auto 0;
}

.expand-close {
    position: absolute;
    top: 10px;
    right: 20px;
    color: #454545;
    font-size: 50px;
    line-height: 50px;
    text-decoration: none;
}

.expand-close:before {
    content: '×';
}

.expand-close:hover {
    color: #fff;
}

.image-large {
    max-width: 100%;
    height: auto;
    display: block;
    padding: 40px;
    margin: 0 auto;
    box-sizing: border-box;
}

@media only screen and (max-width: 530px) {
    .image-cell {
        width: 50%;
    }

    .image-cell:nth-of-type(2n+2) .image-expand {
        margin-left: -100%;
    }

    .image-cell:nth-of-type(2n+3) {
        clear: left;
    }

    .image-expand {
        width: 200%;
    }
}

@media only screen and (min-width: 531px) {
    .image-cell {
        width: 20%;
    }

    .image-cell:nth-of-type(5n+2) .image-expand {
        margin-left: -100%;
    }

    .image-cell:nth-of-type(5n+3) .image-expand {
        margin-left: -200%;
    }

    .image-cell:nth-of-type(5n+4) .image-expand {
        margin-left: -300%;
    }

    .image-cell:nth-of-type(5n+5) .image-expand {
        margin-left: -400%;
    }

    .image-cell:nth-of-type(5n+6) {
        clear: left;
    }

    .image-expand {
        width: 500%;
    }
}