How can I center an image in Bootstrap?

The Bootstrap class .center-block works well.

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.

Since the img is an inline element, Just use text-center on it's container. Using mx-auto will center the container (column) too.

<div class="row">
    <div class="col-4 mx-auto text-center">
        <img src="..">
    </div>
</div>

By default, images are display:inline. If you only want the center the image (and not the other column content), make the image display:block using the d-block class, and then mx-auto will work.

<div class="row">
  <div class="col-4">
    <img class="mx-auto d-block" src="..">
  </div>
</div>

Demo: http://codeply.com/go/iakGGLdB8s

3 ways to align img in the center of its parent.

1.

img is an inline element, .text-center aligns inline elements in center if the element its used on, is a block element.

<link href="https://cdnjs.cloudflare.com/ajax/libs/twitter-bootstrap/4.1.1/css/bootstrap.css" rel="stylesheet"/>
<div class="container mt-5">
  <div class="row">
    <div class="col text-center">
      <img src="https://upload.wikimedia.org/wikipedia/en/8/80/Wikipedia-logo-v2.svg" alt="" class="img-fluid">
    </div>
  </div>
</div>

2.

.mx-auto centers block elements. In order to so, change the display of the img from inline to block using the .d-block class.

<link href="https://cdnjs.cloudflare.com/ajax/libs/twitter-bootstrap/4.1.1/css/bootstrap.css" rel="stylesheet"/>
<div class="container mt-5">
  <div class="row">
    <div class="col">
      <img src="https://upload.wikimedia.org/wikipedia/en/8/80/Wikipedia-logo-v2.svg" alt="" class="img-fluid d-block mx-auto">
    </div>
  </div>
</div>

3.

Use .d-flex and .justify-content-center on its parent.

<link href="https://cdnjs.cloudflare.com/ajax/libs/twitter-bootstrap/4.1.1/css/bootstrap.css" rel="stylesheet"/>
<div class="container mt-5">
  <div class="row">
    <div class="col d-flex justify-content-center">
      <img src="https://upload.wikimedia.org/wikipedia/en/8/80/Wikipedia-logo-v2.svg" alt="" class="img-fluid">
    </div>
  </div>
</div>