.contenedor {
  position: relative;
  height: 90px;
  width: 90px;
  margin: 50px 20px;
  float: left;
}

.contenedor img {
  position: absolute;
  left: 0;
  transition: opacity 1.5s ease-in-out;
 
} 

.contenedor img.top:hover {
  opacity: 0;

}