body {
  display: flex;
  justify-content: center;
  align-items: center;
  background-color: #333;
  margin: 0;
}
.container {
  position: relative;
  width: 150px;
  height: 150px;
}
.image1, .image2 {
  position: absolute;
  width: 300%;
  height: 300%;
  transition: opacity 0.3s;
}
.image2 {
  opacity: 0;
}
.container:hover .image2 {
  opacity: 1;
}
.container:hover .image1 {
  opacity: 0;
}