Oh, well, it might be about the issue that the OP makes in a comment on that post.
For me, the URL of that image is startrek.website/…/ffcc19ac-fb10-4c66-893e-87840b…, which is different from the one in your screenshot.
Viewing feddit.nl/c/risa@startrek.website in Private fixed the thumbnail for me in Normal view, so it suggests it’s a caching issue. Perhaps try to purge your browser’s memory of the community, and see if then comes up with the correct URL.