Image by default is displayed as inline-block, you need to display it as block in order to center it with .mx-auto
. This can be done with built-in .d-block
:
<div class="container">
<div class="row">
<div class="col-4">
<img class="mx-auto d-block" src="...">
</div>
</div>
</div>
Or leave it as inline-block and wrapped it in a div with .text-center
:
<div class="container">
<div class="row">
<div class="col-4">
<div class="text-center">
<img src="...">
</div>
</div>
</div>
</div>
I made a fiddle showing both ways. They are documented here as well.