a {
position: relative;
}
a img:last-child {
position: absolute;
top: 0;
left: 0;
}
a:hover img:first-child,
a img:last-child {
opacity: 0;
visibility: hidden;
transition: visibility 150ms ease, opacity 150ms ease;
}
a:hover img:last-child {
opacity: 1;
visibility: visible;
}