Agreements in black and white are most likely to be observed. It’s logical: a written agreement is evidence. E-mails are perfect for this, but the tone is important. You can write: “I hereby confirm that the photographic material that is subject to our agreement cannot be used outside of the United States without the payment of extra compensation.” But this could be taken as a sign that you don’t trust them. Why not write: “I look back on our fruitful meeting with pleasure. If you would also like to use my pictures outside of the US, I would be pleased to send you an offer for such use.” This keeps the relationship friendly and implies that you are willing to grant further rights. But most importantly, you have clearly implied – in document form – that they only have the rights for the United States, and that if they use the photos elsewhere, they are obligated to pay you extra for this. Confirm agreements using e-mail, and build up a case file.