Maybe I can answer my question by myself: I simply replaced the word crop by product (in the widget), as it seems that the generated product folder contains thumbnails (strangely, the folder "thumbnails" neither contains files).
Unfortunately, the documentation is not explaining much about many widget options (what means "tag which is used to form images in the gallery." which is the explanation of that gallery tag?).
But I see that the quality of docuumentation is improving on
http://seotoaster-documentation.seotoaster.com/ (by the way, the link in the footer of your site still links to the old documentation).
Philippe