The usage is additional from the original agreement. It was an 8 image national campaign and now they want to run 4 images international. The original agreement has been fulfilled, paid and closed. They are asking for additional usage on 4 images to run international.
That's very different then.
You may renegociate the fee for international usage. It should be divided but IMO not based on the first agreement so even dividing it should comes close to the full porcentage.
I would negociate.