WebPShop VS dont-accept-webp

Grab a the webp plugin for Photoshop: https://developers.google.com/speed/webp/docs/webpshop

Knock yourself out mate

Webp is a newer format and adoption hasn't reached universal levels yet, but there's a free (official) Photoshop plugin: https://developers.google.com/speed/webp/docs/webpshop. It compresses well and can store lossless data.

This fixes it

Try the free Squoosh.app from Google, you can see before and after at the same time. Ideally you'd use webp as the format, it's supported by most browser and WP since 5.8. They also have a webp plugin for Photoshop which uses the same algorithm, meaning that you can try with the before and after on their site and test different percentages, and then replicate or automate it in Photoshop.

most common would be to use Photoshop with Google's WebPShop plugin

Here's the one for firefox: https://github.com/jscher2000/dont-accept-webp

25. Don't Accept image/webp 0.8.2 Strip image/webp from Accept: headers, which should discourage servers from replacing JPEG and PNG images with WebP images. https://github.com/jscher2000/dont-accept-webp

Add-ons can intercept requests and modify Accept headers globally (example) or based on parameters added to the URL (example) but I don't know how easily they can define new script methods that you could use in a page, if that is a requirement, or determine that it is the first GET request in a tab, if that is a requirement.

If you're on Firefox there's a tiny add-on that has helped me a lot with this issue: https://github.com/jscher2000/dont-accept-webp

Same here, https://github.com/jscher2000/dont-accept-webp