iDrive cloud backup of photos

Perhaps this question should be posted to a different forum...

As new pictures are added to the "Photos Library.photoslibrary" on my iMac new backups into my iDrive cloud backup service will need to be made. (This is a cloud backup service offered by Apple and is not the same as storing pictures on my iCloud account.) Does the new backup just involve the newly added pictures or is the entire library re-backedup? Doing the entire large library would be very inefficient and potentially take many hours each time.

Thank you.