No. Talk to a copyright lawyer. Immediately.

If the images were obtained through a Google search, it is possible one of the images you used was actually copywrited by Getty Images. Although it sounds like GI didn't follow the proper channels in dealing with this situation (they should have first sent a cease and desist letter clearly identifying what image was used in violation of copyright and proving their copyright) however, the potential exists that failure to respond could result in the company taking legal action against you or your client.

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The best way to learn anything, is to question everything.