The files have metadata to help find the source.(Digimarc used to be implemented also.) The meta data is easily removed as is editing out the watermark.
However those aren't ever going to be paying customers anyway. When you license stock, you get more than just the high resolution file, but also 1. exclusivity if you've requested it and the necessary waivers to use the image in the way intended.
For example, a night time shot of the Eiffel Tower requires a waiver, as the light show is subject to copyright. (Believe it or not!) While a day shot won't require a waiver.
Not having a legitimate waiver is asking for a damages case by the owner, those depicted or other entity. This is covered in most countries via copyright & publicity laws.
However those aren't ever going to be paying customers anyway. When you license stock, you get more than just the high resolution file, but also 1. exclusivity if you've requested it and the necessary waivers to use the image in the way intended.
For example, a night time shot of the Eiffel Tower requires a waiver, as the light show is subject to copyright. (Believe it or not!) While a day shot won't require a waiver.
Not having a legitimate waiver is asking for a damages case by the owner, those depicted or other entity. This is covered in most countries via copyright & publicity laws.