Without explicitly defining the height I determined I need to apply the flex value to the parent and grandparent div elements...
<div style="display: flex;">
<div style="display: flex;">
<img alt="No, he'll be an engineer." src="theknack.png" style="margin: auto;" />
</div>
</div>
If you're using a single element (e.g. dead-centered text in a single flex element) use the following:
align-items: center;
display: flex;
justify-content: center;