If the image was legally obtained, then the person who designed the website for you will have the receipt from the purchase of the image, and should have forwarded that to you when they transferred ownership of the site to you. If they did not forward this to you @ the time of sale, contact them now and request the information immediately. Essentially you need to prove that the image was legally purchased.
I would also consider forwarding the designer's information to Getty as it would appear that the image/design may have been sold to you in bad faith. If the images were stolen, or if the images were represented to you as being OK to use when they were not, that makes you a victim and not a malicious offender. I'm not saying that makes you 100% innocent (it does not) but you can help Getty get closer to someone who is ripping off their property.
Do some digging on your designer and find out the "rest of the story." Then decide at that point what your next move is... getting a lawyer, dealing with Getty yourself, etc. Whatever you do though, don't just ignore them. They will find you, they will take you to court, and they will cause you a world of hurt. Their pockets are a lot deeper than yours and they have nothing to cause them to drop their case (like, they aren't going to run out of money). They aren't going to just go away.