Visual derivative is a preprocessed picture itself, and it's part of the "safety voucher".

It may be grayscale and resized/normalized in other ways. Only apple knows exactly what it is.

It's only a matter of time until internet trolls find a way to abuse this. The database is stored on user's devices, so someone downloading it and permuting innocuous images enough until they match the database, and then spreading them for funzies via some underhanded method (wallpaper download sites?)... is not too far fetched.