Happy to help and glad to hear that you got the webp images to work!
> I have installed the webp codec from Google. It has enabled Windows Explorer to created thumbnails and the various Windows image viewers to load webp images. However has had no effect on ThumbsPlus.
Yes, that makes sense. A Windows codec, originally, is a (de)coder for video files and streams. It hooks into Windows' Directshow interface, which is what ThumbsPlus uses to thumbnail and display videos.
Microsoft later introduced an interface for images called Windows Imaging Component, WIC for short. This uses a different kind of (de)coders, not for movies but for images, and some refer to those as codecs too. ThumbsPlus does not use these WIC-codecs (but maybe it should?).
Anyway, a codec for .webm videos (VP8 or VP9 encoded) is part of the [link=https://www.codecguide.com/download_kl.htm]K-Lite Codec Pack[/link], so if you want to get those videos to work in ThumbsPlus too, that's one possible source.