This is inspired by rules used in wordpress plugin https://github.com/rosell-dk/webp-express
It will serve a webp image to browsers that can handle it.
GET /my/image.jpg will return image previously converted and saved as /my/image.jpg.webp
on debian you might want this package
~# apt install webp
imagemagick / graphicsmagic can do the job too
Hope it helps
Notes
ADDVARY variable has a meaning and without it apache won't add the Vary header on a rewrite rule.
not sure that EXISTING variable is required, but as is it works as expected.
Having the Vary header makes this respects http current standards and makes it compatible to use behind CDN.
In case CDN ignored the Vary, one could add the 'Cache-Control: Private' which explicitly bans reverse proxy caching and still take advantage of browser cache.