Hello,
It would be a quicker process for detecting used images, and let the rest as unused images, when you decide to use API. So, I will create a script which will use API for enumerating catalogs and categories, then identify the image used by them. Then I will create a list of this used images, and then the script will record the rest of files as unused images. This will save much time for whole process.
Best Regards,
Khamid