Euclid Theme – Featured Image Centering

Hey guys,
Thanks for any help in advance as this one is driving me nuts, software coding is not my forte.
Trying to get my featured images to center in posts etc, if the image isnt wide enough to stretch the allowed length of 610px.

Take e.g
https://rattlehead.ie/?p=421 . Trying to get the image to center doesn’t seem to work.

While https://rattlehead.ie/?p=342 . As its big enough this works.

I dug down into inspecting the element and found the below code in my Euclid theme.

.post .featured-image {
width: 610px;
height: 350px;
}

I thought maybe adding a position:center; or something else might work, but no joy. Infact I can’t be 100% sure Im looking in the right place. It is the right .css file, but Im not sure if its taking the attributes from another class??!!

Below is the element inspect snippet

<div class="featured-image">
            <img width="300" height="300" src="https://rattlehead.ie/wp-content/uploads/2010/12/rattlehead.jpg" class="attachment-post-standard wp-post-image" alt="Rattlehead">
            <div class="post-icon">
                <span class="fa-stack fa-lg">
                    <i class="fa fa-circle fa-stack-2x"></i>
                    <i class="fa fa-pencil fa-stack-1x fa-inverse"></i>
                </span>
            </div>
            </div>

As I said, any help or pointers would be a great help.