I am guessing you are using "Page previews":
https://www.mediawiki.org/wiki/Page_Previews
(This has to be enabled by each user on
https://fr.wikipedia.org/wiki/Spécial:Préférences under "Aperçus de page".)
The tool loads its data from here:
https://fr.wikipedia.org/api/rest_v1/page/summary/Pince_à_avoyer
https://fr.wikipedia.org/api/rest_v1/page/summary/Pince_universelle
The first of these pages has a "thumbnail" defined, the second doesn't.
Unfortunately I don't know what generates this data, but perhaps someone
else can help.
--
Bartosz Dziewoński
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