.thumbnail {
    position: relative;
    display: inline-block; /* occupies height of the small image */
    z-index: 0;
}

.thumbnail:hover {
    z-index: 50;
}

.thumbnail img:nth-child(2) { /* CSS for enlarged image */
    position: absolute;
    visibility: hidden;
}

.thumbnail:hover img:nth-child(2) { /* CSS for enlarged image on hover */
    visibility: visible;
    top: 20px;  /* offset enlarged image down */
    left: 60px; /* offset enlarged image right */
}