how to pull image into center in bootstrap code example

Example 1: bootstrap 4 center image

/* used mx-auto d-block */

<img class="mx-auto d-block" src="https://www.w3schools.com/bootstrap4/paris.jpg" alt="https://www.w3schools.com/bootstrap4/paris.jpg"/>

Example 2: html center image vertically bootstrap

mx-auto mt-auto mb-auto