.wpa9-gallery-image-container {

}

  .wpa9-gallery-image-container .gallery.gallery-columns-2-first-big .gallery-item:first-child {
    width: 100%;
  }

  .wpa9-gallery-image-container .gallery.gallery-columns-3-first-big .gallery-item:first-child {
    width: 100%;
  }