Why WebP will not open
WebP is a newer format. Some forms, older programs, printers and chat apps still reject it, which is why a saved web image sometimes cannot be uploaded.
Convert with PicFitter
Open the WebP to JPG converter, choose your pictures (several at once is fine) and download the JPG files. It runs in your browser, so nothing is uploaded.
Without a tool
Open the WebP image in a browser, right-click and look for "Save as" with another format, or open it in an editor that supports WebP and export as JPG.
Notes
Transparent areas become white in a JPG. If you need transparency, convert to PNG with the WebP to PNG tool instead.