Realistically, I'd supply the photos and adjust the contract moving forward.
My reasoning is that it is simply good business, and it will make you look good. Not to mention, they obviously value your product -- maintaining relationships with clients, IMO, can be a good thing.
You mention a networking exchange. I'm not entirely sure what that means, but, it isn't a bartering agreement - nor is it a social call. Obviously not a gift, nor was it a situation where you were offered a % of their profits.
Sure, shame on you both for not establishing expectations. Kudos to you for recognizing a learning experience for what it is.
In this situation I'd probably acknowledge that expectations were not defined (ie; budgets or costs), and offer a 12% discount -or- the option to decline your product entirely without any questions asked or hard feelings.
When my neighbor gives me a ride down the street to pick up my car at the mechanic, I slip them a $20. When another friend took down a very small pine tree, I paid for their chainsaw - and then some. I want people to be fully compensated for the things they do for me.