I have PNGs on my site as well and I don’t see it as an issue. I used a plugin to optimize my site images using JCH Optimize: https://ww.wp.xz.cn/plugins/jch-optimize/
It somewhat does the job in improving your overall GT Metrix scores plus I have CloudFlare CDN enabled on my site. It’s a basic landing page though.
https://www.screencast.com/t/jhdLvdsb
I was optimizing my site and I bumped into the same “Serving Scaled Images” issue. I found this article and it worked pretty well for me, this might help:
https://www.hostinger.ph/tutorials/website/improving-website-performance-serving-scaled-images
