@media screen and (min-width: 30em) {
  .gallery {
    display: flex;
    flex-flow: column nowrap;
  }
  .gallery > * {
    width: 50%;
  }
  .gallery > *:nth-child(even) {
    align-self: flex-end;
  }
}
@media screen and (min-width: 64em) {
  .gallery {
    position: absolute;
    top: 5rem;
    left: 40rem;
    width: 30rem;
  }
}