I seem to have an issue on the big-thumb feature for single posts. For some reason I have a 5px bottom-border. ( Here is a link to show what I mean: http://imgur.com/a/CloSn )

This happens regardless of what image size I upload as the featured image. I even tried using exactly the same image, title and intro from the theme-demo here:

About an powerful connections humans have with the ocean

But the same thing happens, still the border?

When viewing from a mobile device, the border is much more evident. I am not using any plugins or custom CSS other than Jetpack for the related posts.