<div class="gallery">
  <img src="https://assets.codepen.io/1506195/unsplash-1.jpg" alt="" />
  <img src="https://assets.codepen.io/1506195/unsplash-7.jpg" alt="" />
  <img src="https://assets.codepen.io/1506195/unsplash-3.jpg" alt="" />
  <img src="https://assets.codepen.io/1506195/unsplash-2.jpg" alt="" />
  <img src="https://assets.codepen.io/1506195/unsplash-6.jpg" alt="" />
  <img src="https://assets.codepen.io/1506195/unsplash-0.jpg" alt="" />
  <img src="https://assets.codepen.io/1506195/unsplash-8.jpg" alt="" />
</div>

body {
  background: #eee;
}

.gallery {
  --size: min(60vmin, 400px);
  position: absolute;
  top: 50%;
  left: 50%;
  transform: translate(-50%, -50%);
  box-shadow: 
    0 0 10px #0002,
    0 20px 40px -20px #0004;
  width: var(--size);
  height: var(--size);
  background: #fff;
  border: 6px solid #fff;
  display: grid;
  grid-template-rows: 50% 50%;
  grid-template-columns: 1fr 1fr;
  overflow: hidden;
  gap: 6px;
}

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

@keyframes moveHorizontal {
  to {
    object-position: 100% 0;
  }
}

@keyframes moveVertical {
  to {
    object-position: 0 100%;
  }
}

@keyframes shrinkVertical {
  to {
    height: 0;
  }
}

@keyframes shrinkHorizontal {
  to {
    width: 0;
  }
}


@keyframes growHorizontal {
  to {
    width: 100%;
  }
}
@keyframes growHorizontal2 {
  to {
    width: 70%;
  }
}

@keyframes growVertical {
  to {
    height: 100%;
  }
}


@keyframes growAll {
  to {
    width: 100%;
    height: 100%;
  }
}

.gallery img:nth-child(1) {
  grid-column: 1;
  grid-row: 1;
  justify-self: end;
  animation: 
    moveHorizontal 8.5s 0.5s 1,
    shrinkHorizontal 2s 9s ease-in 1;
  animation-fill-mode: forwards;
}

.gallery img:nth-child(2) {
  grid-column: 2;
  grid-row: 1;
  justify-self: end;
  animation: 
    shrinkHorizontal 2s 11s 1;
  animation-fill-mode: forwards;
}

.gallery img:nth-child(3) {
  grid-row: 2;
  grid-column: 1 / 3;
  align-self: end;
  object-position: 0 0;
  animation:
    moveVertical 5s 1s 1,
    shrinkVertical 3s 5s 1;
  animation-fill-mode: forwards;
}

.gallery img:nth-child(4) {
  grid-column: 1 / 3;
  grid-row: 1;
  width: 0;
  justify-self: center;
  align-self: start;
  animation: 
    growHorizontal 2.25s 11s 1,
    moveHorizontal 4s 14s 1,
    shrinkVertical 2s 18s 1;
  animation-fill-mode: forwards;
}

.gallery img:nth-child(5) {
  grid-column: 1;
  grid-row: 2;
  width: 0;
  justify-self: start;
  align-self: end;
  animation: 
    growHorizontal 2.5s 7.5s 1,
    moveVertical 4s 12.5s 1,
    shrinkHorizontal 2s 17s 1;;
  animation-fill-mode: forwards;
}

.gallery img:nth-child(6) {
  grid-column: 2;
  grid-row: 2;
  width: 0;
  justify-self: end;
  align-self: end;
  animation: 
    growHorizontal 2s 8s 1,
    shrinkHorizontal 2s 17s 1;
  animation-fill-mode: forwards;
}

.gallery img:nth-child(7) {
  grid-column: 1/3;
  grid-row: 1/3;
  width: 0;
  justify-self: end;
  align-self: end;
  object-position: 0 0;
  animation: 
    growHorizontal 2s 20s 1,
    moveHorizontal 16s 21.5s 1;
  animation-fill-mode: forwards;
}