Many websites use WebP because it’s usually smaller than PNG. A PNG becomes one without losing quality. Rename the file in Finder and Renamable converts it right where it is. Nothing is uploaded.
PNG.png
WebP.webp
Screenshot.pngScreenshot.webp
Yes
How to do it.
Select Screenshot.png in Finder and press Return.
Change .png to .webp and press Return. If Finder asks, choose “Use .webp”.
Renamable converts it right there. The original goes to the Trash, or stays next to the new file if you prefer, and you can undo any conversion.