I've been seeing lots of broken thumbnails lately. Typically re-uploading the image will fix it. Here's an unfixed example:
https://en.wikipedia.org/wiki/Esmeraldas_woodstar, which contains the broken thumbnail:
(Update: This is now working for the reporter.)
Interestingly, clicking that 2nd link directly causes my browser to download the image (rather than display it), which makes me think it's possibly an issue with the headers. The thumbnail itself (once downloaded) displays fine locally.
I can't reproduce the bug in Firefox or Safari, only Chrome, so it may be related to a particular caching server or a browser issue or an issue local to my machine. Will wait to see if anyone else can reproduce.
Using Chrome 76.0.3809.100 on MacOS.