picture box bootstrap 4 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: how to make image responsive bootstrap 4
<img src="..." class="img-fluid" alt="Responsive image">