Hi Kelvin. The white borders are coming from from your theme. I can see you’re not currently using the slideshow but I can see them on your header image. Adding the CSS below to your theme’s style.css file should get rid of them.
Very sorry about that, don’t know how I missed it! No, the same information doesn’t apply.
What’s actually happening is that the slideshow is contained within <pre> and <code> HTML elements causing the styling. You need to remove these by changing to the “Text” editor when editing a Post (it is probably set to the “Visual” editor tab).
Then, remove the <pre>, </pre>, <code> and </code> HTML.