.image-grid div {
  min-height: 200px;
  background-repeat: no-repeat;
  background-position: center;
  background-size: cover;
}
@media (min-width:768px) {
  .image-grid div {
    min-height: 300px;
  }
}
@media (min-width:1200px) {
  .image-grid div {
    min-height: 400px;
  }
}

@media (min-width:1400px) {
  .image-grid div {
    min-height: 500px;
  }
}