With Piwigo 2.4.0 and 2.4.1 and might be on going versions, Thumbnails are missing in WordPress widget.
The main reason could be the new cache used by Piwigo.
I am looking for a smart corrective.
Please be patient for the time being.
If you like, I could send you a zip of the new version.
Comment with a valid email the related topic on my blog.
PiwigoPress 2.0.0 includes:
* Support of WordPress from 2.8.0 to 3.4.1 (and probably above)
* Support of Piwigo 2.4.x (and probably above)
* Support of Piwigo 2.0.x – 2.3.x assumed
* cURL access support (3rd way to solve webservice call issues)
* CSS DIV class: img-shadow and/or img-show-desc are now provided
* Orientation filtering