If you embed Instagram posts using the Instagram embed code, for some reason Instagram makes it always auto align to the very left of the page.
I used the tips on this site to make it stop going all the way to the left:
The code is here (you can add this to Code injection to your site)
<style>
/* Stop Instagram auto-aligning to the left */
.instagram-media{ margin:0 auto !important; }
</style>
I embed Instagram posts in Ghost by using the HTML card:
You can see it in action here: