position images in center of container

body {
  margin: 0;
}

#over img {
  margin-left: auto;
  margin-right: auto;
  display: block;
}

.image-container {
    width: 500px;
    height: 500px;
    position: relative;
}

.image-container img {
    position: absolute;
    right: 0;
    left: 0;
    top: 0;
    bottom: 0;
    margin: auto auto;
}