<div class="gallery">
  <img src="https://picsum.photos/id/1028/400/400" alt="a forest after an apocalypse">
  <img src="https://picsum.photos/id/15/400/400" alt="a waterfall and many rocks">
  <img src="https://picsum.photos/id/1040/400/400" alt="a house on a mountain">
  <img src="https://picsum.photos/id/106/400/400" alt="sime pink flowers">
  <img src="https://picsum.photos/id/136/400/400" alt="big rocks with some trees">
  <img src="https://picsum.photos/id/1039/400/400" alt="a waterfall, a lot of tree and a great view from the sky">
  <img src="https://picsum.photos/id/110/400/400" alt="a cool landscape">
  <img src="https://picsum.photos/id/1047/400/400" alt="inside a town between two big buildings">
  <img src="https://picsum.photos/id/1057/400/400" alt="a great view of the sea above the mountain">
  <img src="https://picsum.photos/id/1016/400/400" alt="a great view of a canyon">
  <img src="https://picsum.photos/id/1015/400/400" alt="a great view of a river between mountains">
  <img src="https://picsum.photos/id/1020/400/400" alt="a wild bear appeared">
</div>

.gallery {
  --n: 3; /* number of rows*/
  --m: 4; /* number of columns */
  --g: 10px;  /* control the gap */
  --f: 1.2;   /* control the scale factor */
  
  display: grid;
  gap: var(--g);
  width: 100vw;
  height: 100vh;
  grid-template-columns: repeat(var(--m),auto);
}

.gallery > img {
  width: 0;
  height: 0;
  min-height: 100%;
  min-width: 100%;
  object-fit: cover;
  cursor: pointer;
  filter: grayscale(80%);
  transition: .35s linear;
}

.gallery img:hover{
  filter: grayscale(0);
  width:  calc(100vh*var(--f)/var(--n));
  height: calc(100vw*var(--f)/var(--m));
}


body {
  margin: 0;
  background: #60c4ff;
}