Unfortunatly Getty are very likely to follow up their demand with court action, they are I believe the Worlds largest holder of images.
I would you write and make an offer stating you were unaware of the infringement of their copywrite.
They will have had to pay the photographer/original owner for use and like any stock photograph organisation also have to pay the owner every time the photograph is used the fee is from £50.00 upto thousands depending on the importance of the image.
Making the excuse that the photograph came from a free image website is certainly not an option, as they would know this isnt true.