<a href="#firstimg"><img src="img/thumb-3.jpg"></a>
<a href="#secondimg"><img src="img/thumb-4.jpg"></a>

<a href="#" class="lightbox" id="firstimg"><img src="img/image-3.jpg"></a>
<a href="#" class="lightbox" id="secondimg"><img src="img/image-4.jpg"></a>

.lightbox {
    display: none;
    position: fixed;
    top: 0;
    left: 0;
    z-index: 2;
    width: 100%;
    height: 100%;
    text-align: center;
    background: rgba(38, 38, 38, 0);
    transition: background 0.3s ease;
}

.lightbox img {
    max-height: 90%;
    max-width: 70%;
    margin: 3%;
    border-radius: 5px;
    border: 5px solid #cbcbcb;
}

.lightbox:target {
    display: block;
    background: rgba(38, 38, 38, 0.7);
}