Hi Topher and Patrick,
Thanks for reaching out to us! I’d be happy to help you.
I looked at your website, and it seems all the thumbnails in the Japanese version have a little problem — not only the ones from the Upcoming Events widget, but also the ones from the posts list.
Could you switch to a default theme, such as Twenty Seventeen, to see whether the problem is related to your theme or not?
Otherwise, there might be a conflict between some plugins. I would recommend you to follow those instructions to test for conflicts.
Please note that the process for testing can result in some amount of disruption to your site. To avoid this, we recommend you either make use of a staging site and test things there or, if that is not possible, consider using a plugin like Health Check which provides a troubleshooting mode that aims to ensure your site will continue to operate as normal for regular visitors.
I hope this helps,
Cheers,
Jeremy