a {
    display: inline-block;
    border: 1px solid #ddd;
    border-radius: 4px;
    padding: 5px;
    transition: 0.3s;
}

a:hover {
    box-shadow: 0 0 2px 1px rgba
    (0, 140, 186, 0.5);
}

<a href="https://cdn.luogu.com.cn/upload/image_hosting/aet06hc8.png">
  <img src="https://cdn.luogu.com.cn/upload/image_hosting/aet06hc8.png" alt="mario">
</a>