Thank you!
I'm realizing that I didn't really explain it anywhere. I should probably update that at some point.
When you first open the game, it will create a custom_images and a custom_word_lists folder. For images, all you need to do is put it in the custom images folder.
For words, you can add a .txt file in the custom_word_lists subfolders long_segments and short_segments (they will be added respectively to either being long word choices or short word choices). And each new line in the .txt is a new word.
After that, in the options you can toggle on your custom words in the Words tab.
Please let me know if you have any other questions!