The "character.txt" file should be saved in the same folder as the voicebank. Make sure it's in Shift-JIS encoding, especially if you're using Japanese characters anywhere in the file. If you're editing the text file in Notepad, selecting "ANSI" will automatically use Shift-JIS if your computer is in Japanese locale.
The sample audio should be a WAV file in the voicebank folder.
The image should be sized 100 by 100 pixels, and saved as a 24 bit BMP file.
For any additional information about your voicebank, such as terms of use, you'll want to create a separate text file named "readme.txt". This will show up in the bottom area of the voicebank information window. You can get started by generating a readme with this website:
https://tools.tubs.wtf/vbtougen