White space when using ‘box-shadow’

Hi, I am using some custom CSS to create a border around my images:

a img {
margin: 15px;
border-radius: 5px;
box-shadow: 0 0 0 5px #6495ED;
}

This works fine on all images within the site, but for some reason on the home page, the featured images have a white space at the top of the box. Is there something I need to change in the stylesheet to prevent this?

Thanks

The page I need help with: [log in to see the link]