This has probably happened because you have renamed the application to something else and the path to the icons can no longer be found.
If you revert back to the original name, it will work again.
Happy with this Captcha, works OK overall in situations when you cannot use Google ReCaptcha for example.
Woops, that was actually a bug! I've fixed it on the newer version.
You should be able to rename the module without any problems on the newer version.