/* (A) GALLERY CONTAINER */
/* (A1) ON BIG SCREENS */
.gallery {
  display: grid;
  grid-template-columns: repeat(3, auto); /* 3 IMAGES PER ROW */
  grid-gap: 10px;
  max-width: 1200px;
  margin: 0 auto; /* HORIZONTAL CENTER */
}
/* (A2) ON SMALL SCREENS */
@media screen and (max-width: 640px) {
  .gallery {
    grid-template-columns: repeat(2, auto); /* 2 IMAGES PER ROW */
  }
}

/* (B) THUMBNAILS */
.gallery img {
  width: 100%;
  height: 200px;
  cursor: pointer;
  /* FILL, CONTAIN, COVER, SCALE-DOWN : USE WHICHEVER YOU LIKE */
  object-fit: cover;
}
.gallery img:fullscreen { object-fit: contain; }

/* (X) DOES NOT MATTER */
body, html {
  padding: 0;
  margin: 0;
}