Good afternoon Neil and welcome back!
Thank you for reaching out to us.
I would love to help you with this topic.
The best way to handle this is through CSS.
Try adding the following CSS rule to your style.css file or in your Custom CSS metabox:
.post-thumbnail img, .wp-post-image, img.size-full, img.size-large {
margin: 0px auto;
display: block !important;
}
If you are looking to limit that rule to desktop only, you might have to wrap the rule inside
@media only screen and (max-width : 992px) { }
Let me know how that goes.
Best regards,
Geoff B.