it's pretty easy to create a .t3x file, you just need to have devkitpro with the tools for 3ds
i'll tell you something curious? RAI Loader allows you to have multiple images for the same character, they are chosen randomly every time you load the character.
This little guide made for Universal Updater works perfectly to make .t3x files for RAI Loader (just remember that the .t3x file must have exactly the same name as the JSON file)
https://github.com/Universal-Team/Universal-Updater/wiki/sprite-sheet