Unable to Further Compress a Single Image in Media Library

Good morning,

I recently purchased and installed WP-Smush-Pro on my newest client site and ran all images through it. It saved 11.7MB! Very nice ~ thank you! However, in doing performance testing there is one image that GTMetrix wants further compressed and I am unable to further smush it via Media Library. It's important because it is the global background image across the site. Can you help?

Thank you!
Michelle