It appears that non-free images (which are stored locally) are not visible in Preview on the desktop site, if you are using Firefox 68.7 ESR on Debian 11.
Reported by @Editor-1, who uses Debian 11, Firefox 68.7 ESR. The problem is reproducible in both safemode and in a private browsing window.
The issue being complained about is that https://en.wikipedia.org/wiki/Skatetown,_U.S.A. has a non-free image in the infobox:
If I hover over that in https://en.wikipedia.org/wiki/Patrick_Swayze#Film I see no image in the page preview:
If I go to https://en.wikipedia.org/wiki/Skatetown,_U.S.A.?action=info I see a page image:
and that image is present in https://en.wikipedia.org/api/rest_v1/page/summary/Skatetown%2C_U.S.A.
but it's not ouputted inn the page preview despite the stance in T147317:
This happens regardless of browser.