pThumb wasn't working with Chinese characters in filenames so I did some troubleshooting and found the culprit is PHP's pathinfo() function.
To fix, upgrade your version of PHP and set your locale in the MODX system settings to include UTF-8.
Mine is: zh_HK.UTF-8
See closed issue here:
https://github.com/modxcms/pThumb/issues/42
Hope this saves someone some time