overflow

<div class="thumb">
  <img src="https://images.pexels.com/photos/33492/cat-red-cute-mackerel.jpg?auto=compress&cs=tinysrgb&h=480&w=640" alt="">
</div>

* {
  box-sizing: border-box;
}

img {
  display: block;
  max-width: 100%;
}

.thumb {
  border: 10px solid green;
  border-radius: 50px;
  width: 480px;

  overflow: hidden;
}