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; 
}