table {
    width: 100%;
}
td {
    width: 20%;
    position: relative;
}
td:after {
    content: '';
    display: block;
    margin-top: 100%;
}

#thumbnail-image-div {
    position: absolute;
    top: 0;
    bottom: 0;
    left: 0;
    right: 0;

    background-position:center center;
    background-repeat:no-repeat;
    background-size:cover;
}

#overlay {
    position: fixed;
    display: none;
    width: 100%;
    height: 100%;
    top: 0;
    left: 0;
    right: 0;
    bottom: 0;
    background-color: rgba(0,0,0,0.5);
    z-index: 1;
    cursor: pointer;
}
#overlayImg {
    margin: 0;
    position: absolute;
    top: 50%;
    left: 50%;
    transform: translate(-50%, -50%);
    height: 500px;

    z-index: 1;
}

.gallery-div {
    width: 75%;
    
    display: flex; 
    justify-content: center;
    
    align-items: center;
    
    z-index: 0;
}