div with img fluid 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: image responsive bootstrap 4
<img class="img-fluid" src="" alt=""> //This will make your image responsive