There is a download link under options for pictures in "theater" mode. I haven't run into any WebP images yet, so I'm not sure if it downloads as jpg or not, but I've read it gives you the jpg link.
In the case of the images linked in the CNet article (https://www.facebook.com/photo.php?fbid=457095677701847&...), it gives you a download link for the jpeg. However, the images in that album are displayed as jpeg (for me), and the only way to view the .webp image is to open in a new tab and add the .webp extension in the url myself.