<a href="javascript:;">
  마우스를 올려보세요. 
  <img src="http://placekitten.com/800/500" alt="">
</a>

a{display:block; width:300px; height: 300px; position:relative; line-height:300px; text-align:center;}
img{position:absolute; top:0; left:0; width:300px; height:300px; object-fit:contain; opacity:0; transition: all 0.5s;}
a:hover img{opacity:1 }