I had a template designer use a Getty Image without an appropriate license, and Getty came after me wanting 4,000 in 'compensation' for the improper use and improper licensing of their intellectual property. They escalated from email messages, to letters, and finally phone calls. They used all kinds of threats to get me to pay.
I found lots of good information on this web site:
http://www.extortionletterinfo.com/
You will find that there are many stock photo companies doing the same copyright trolling. Its a entire sideline business and source of cash flow for stock photo galleries. I sell stock image I take, and I've yet to get paid any compensation for the photos that have been used without license.
Do take the email seriously. Respond, but DO NOT admit any guilt. While Getty's lawyers make some extreme threats, in reality, Getty has yet to go to court to force a settlement. If you defend your rights vigorously, it will cost Getty's lawyers a lot of time and effort, which means a lot of expense in legal fees to Getty. The more effort, the more cost, the less likely they will go after you. If they DO go to court, your odds are pretty good that an impartial judge will knock an unintentional violation of copyright down to 400 bucks in damages and legal fees to the infringed party, so, Getty won't likely go to court and take their chances as they will lose more than it cost them to get to court and get a settlement.
Now, keep in mind, I'm not a lawyer. You may want to consult your lawyer to make sure you are within your rights.
Take down the offending images immediately so as to show your intention not to willfully violate copyright. It may also limit any damages if in the event you are found guilty. Don't offer to pay anything, but be cooperative in responding to messages, emails, letters, phone calls but do not admit guilt or offer to pay anything as you have rights too. Remember, these people contacting you work for Getty and have Getty's best interests in mind (and action). They will say anything to get you to pay, including lie and mislead you. Some of these copyright trolls are paid a percentage of what they collect, so if they don't collect, they don't get paid, which is another matter entirely for the Bar Association to consider if you wish to file a complaint with them regarding the lawyer that is contacting you's conduct. As I said, consult your own attorney to find out your rights, and then stand behind them.
Good luck, and don't buy or license any image Getty or any of their other brands (Like IstockPhoto Dot Com). Boycotting them is the best revenge.