.gallery {
    display: flex;
    flex-wrap: wrap;
    max-width: 100%;
}

.gallery-item {
    width: 25%;
    padding: 1px;
}

    .gallery-item > a > img {
        width: 100%;
        height: 200px;
        padding: 2px;
        border: 1px solid #C6C6C6;
        object-fit: cover;
        transition: .5s ease-in-out;
        display: block;
    }

        .gallery-item > a > img:hover {
            border: 1px solid #9E9E9E;
        }

img {
    opacity: 1;
    transition: opacity 1s;
}

    img[data-src] {
        opacity: 0;
    }