I think people just believe it is their "right" to have the photos and just don't want to pay. I would probably send them a bill for what you charge for the advertising file. Put a note in there that nicely states, "I am sure it is just an oversight and your bill for the advertising file is X$$$. Thank you for your business".

This will give them a way out of what they KNOW they have done. Don't know if it will work, but, it is a shot.

Very few people are honest these days