<div class="flex-container">

  <img src="https://upload.wikimedia.org/wikipedia/en/7/77/EricCartman.png" alt="">
  
  <img src="https://upload.wikimedia.org/wikipedia/en/7/77/EricCartman.png" alt="" class="self-center">
</div>

.flex-container {
  display: flex;
  flex-direction: column;
  justify-content: center;
  width: 100%;
}

.self-center {
  align-self: center;
}