As soon as I read your letter I immediately removed the image from our website andmade sure no copies existed on the computer or backup drives.
Maybe you could word it to say you modified the html of your pages to ensure the images in question no longer appear when your pages are display. This leaves the question open whether the images were were stored on your server.
But bear in mind: If you did store the images on your server and Getty has gotten some smarts they will have started to store html. In which case, they will know you had the image on your server. (Still, don't say actively admit it. If they want to tell you they know it was on your server, let them say so.)
Also: Do not tell them where you "get" your images. None of that information needs to be shared with them. But either now-- or when you get the next letter-- be ready to ask them for proof that they have a right to represent the copyright owner. So: registration, contract and so on.
Also, in the meantime, it can be useful to learn how to look up registrations at the US copyright office. (It's not that hard.) In fact it's useful to do this before sending any letter. (We can explain how. But it's late-- so if you need help, ask, and I can point you to the place tomorrow.)