It seems to be an intermediary caching issue between me and Miraheze's servers, as RhinosF1 and paladox have checked everything with their servers, or it could, potentially, be a MediaWiki-related software issue as I have encountered this on English Wikipedia, but I'm wondering if you might want to try something. Instead of overwriting the image with the a black thumbnail (which I, and potentially other Canadians, can't see as we still see the overwritten image), would you be able to try deleting the page(s) and creation protecting the titles limited only to administrators on this wiki? Or is there a specific reason why you prefer to simply overwrite the image? Not sure of your local procedures, but this, I suspect, may be a workaround.
Cheers,
Doug