Image margins – making them full-width

Hi everyone!

This is my first time on the forum.

I’m having trouble making the width of an image span the entire width of the post.

This is the problem page:

thewrittenlifeofwalt.com/how-we-read-and-review

The image is at the bottom, and I want it to be as wide as the entire post.

But of course I want to retain the margins of the text.

If there’s anyone who knows how to do this, it would be greatly appreciated.

Glenn