Update by Spatulade:

jsonner_new uses the python package Pillow to extract the offsets for a folder that doesn't contain a filelist txt (by taking the bottom-middle of the largest sprite in the folder). If you have any issues, use jsonner_old instead for the classic version which just sets the offsets to 999.

Creates a JSON file for custom sprites and custom images.

To use, download the KeeperFX GFX Archives. Then put the extracted files in the correct folder and run jsonner.bat to get the json you want.
The JSON will hold the folder structure as present in that folder, and the sprite offsets are automatically set from the filelist txt files inside.

Example:
If you want to make a custom ghost, you want to download the default ghost and edit the images of the original ghost, and put that back in the game as a custom sprite.
You download and extract the gfx files then navigate to the /enginefx/sprites-32/creatures/creature_ghost/ folder inside, and put jsonner there. Double click it and get a json file for a ghost.

This application requires Python to be installed on your PC. Download and install from here: https://www.python.org/downloads/windows/

The application itself just needs to be extracted and does not require any further installation.