This is great stuff! I hope Google decides to add some kind of carbon calculator to Lighthouse: we can't fix what we don't measure.
For what it's worth, the Core Performance Team at wordpress.org offers a Performance Lab plugin to do stuff like convert images to the lighter-weight webp format, and many other things. wordpress.org/plugins/performance-lab
(Yeah, I know, WordPress is old tech. But, it's not dead yet. It still powers something like 40% of the websites on the web by website count not traffic. So there's a LOT of carbon-saving leverage to be had by making it more efficient.)