Why does flexbox stretch my image rather than retaining aspect ratio?

Question

It is stretching because align-self default value is stretch. Set align-self to center.

align-self: center;

See documentation here: align-self

The key attribute is align-self: center:

.container {
  display: flex;
  flex-direction: column;
}

img {
  max-width: 100%;
}

img.align-self {
  align-self: center;
}

<div class="container">
    <p>Without align-self:</p>
    <img src="http://i.imgur.com/NFBYJ3hs.jpg" />
    <p>With align-self:</p>
    <img class="align-self" src="http://i.imgur.com/NFBYJ3hs.jpg" />
</div>

I faced the same issue with a Foundation menu. align-self: center; didn't work for me.

My solution was to wrap the image with a <div style="display: inline-table;">...</div>

Adding margin to align images:

Since we wanted the image to be left-aligned, we added:

img {
  margin-right: auto;
}

Similarly for image to be right-aligned, we can add margin-right: auto;. The snippet shows a demo for both types of alignment.

Good Luck...

div {
  display:flex; 
  flex-direction:column;
  border: 2px black solid;
}

h1 {
  text-align: center;
}
hr {
  border: 1px black solid;
  width: 100%
}
img.one {
  margin-right: auto;
}

img.two {
  margin-left: auto;
}

<div>
  <h1>Flex Box</h1>
  
  <hr />
  
  <img src="https://via.placeholder.com/80x80" class="one" 
  />
  
  
  <img src="https://via.placeholder.com/80x80" class="two" 
  />
  
  <hr />
</div>