I would recommend you look in to the margin property, as I was able to centre your image using that.
On another note, please ensure you have the test suite on your project page, as it needs to be present with all tests passing when you submit. When I looked at your page, the script was not present for me. You can add <script src="https://cdn.freecodecamp.org/testable-projects-fcc/v1/bundle.js"></script> to the bottom of your HTML code to do this.