.promo-block img {
  width: 100%;
  height: 100%;
  object-fit: cover;
}

.promo-block img + img {
  display: none;
}

@media (min-width: 750px) {
  .promo-block img:first-child:not(:only-child) {
    display: none;
  }

  .promo-block img + img {
    display: block;
  }
}