That will not work as the text-align property applies to block containers, not inline elements, and img is an inline element. See the W3C specification.

Use this instead:

img.center {
    display: block;
    margin: 0 auto;
}
<div style="border: 1px solid black;">
<img class="center" src ="https://cdn.sstatic.net/Sites/stackoverflow/company/img/logos/so/so-icon.png?v=c78bd457575a">

</div>