I second the recommendations of
darkhound1.
JPEG blurs color borders, so the image quality lowers significantly. WEBP gives practically no perceptible decrease in quality (on good quality settings like 90%), while being also superior in size reduction. So there is no question, WEBP it is.
For batch image processing, there is also XnConvert from the same author as XnView, and even a command-line tool to be run from automatic scripts.
Sorry for being late to the party and bringing this up again but your comparison is an oversimplification and the resulting conclusion sadly not sound.
Both JPEG as well as WEBP have image details they struggle with: JPEG suffers on hard borders of uniform color or small lines on uniform backgrounds, WEBP suffers on areas of small details like leaves on a tree or human skin. Given a modern encoder like
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the size difference between both formats on equally perceived quality is negligible and depending on image composition JPEG may still draw ahead. So the question still remains and it's answer isn't always WEBP.
That being said, I too second
darkhound1's recommendation of using fixed quality WEBP over most JPEG encoders,
CrimsonAxisStudio. Though if one tried to optimize for size as well as quality a structural-similarity-based approach using variable quality settings for each image would be the way to go. And while the process may sound rather daunting, it can be automated by tools like
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.
I actually did test this on a relatively new title last autumn which shipped both a PNG- as well as a JPEG-version (the latter with a quality setting of 90, iirc) on its first iterations:
