header, div {
  text-align: center;
}

main {
  display: flex;
  flex-wrap: wrap;
}

div {
  height: 40vh;
  flex-grow: 1;
}

img {
  max-height: 100%;
  min-width: 100%;
  max-width: 100vw;
  object-fit: cover;
  vertical-align: bottom;
}