align image center in bootstrap 4 code example

Example 1: image responsive bootstrap 4

<img class="img-fluid" src="" alt=""> //This will make your image responsive

Example 2: bootstrap Aligning images

Align images with the helper float classes or text alignment classes.
block-level images can be centered using the .mx-auto margin utility class.

<img src="..." class="rounded float-left" alt="...">
<img src="..." class="rounded float-right" alt="...">
<img src="..." class="rounded mx-auto d-block" alt="...">