Responsive image
<img src="..." class="img-fluid" alt="Responsive image">
...
<img src="..." alt="..." class="img-thumbnail">
... ...
<img src="..." class="rounded float-left" alt="...">
<img src="..." class="rounded float-right" alt="...">
...
<img src="..." class="rounded mx-auto d-block" alt="...">
...
<div class="text-center">
    <img src="..." class="rounded" alt="...">
</div>