img center in div bootstrap 4 code example
Example 1: bootstrap class align center image
class="mx-auto d-block"
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="...">