The nature of 5eTools means that from time to time the service may be offline or that you (as a user) may wish to take the code and host it locally. This tool exists to allow the script to update the image URLs en masse if the image repository is offline. (When the image repository is down, all the tokens, portraits, maps, and handout images are broken and so are not usable.)
All the images' addresses being modified have to be in the same folder structure and have the same filenames.
Support/Suggested Use Guidance.
Don't use this without being well prepared.