There are basically 3 strategies you can use:
Ignore the letter. I agree this is a very reasonable strategy if you have a thick skin and are not bothered by threats from lawyers. The fact that you are bothered by the threat from the lawyer tells me that is not the best strategy for you. This generally ends up with Getty dropping the matter and you paying them nothing. But if they take you to court, you will pay many thousands of dollars. Do you feel lucky today.
Handle it yourself. Pay the settlement or try to negotiate the amount down. That won't stop the hassles, and may even increase them as they then know they have you worried. In this case you said the demand was $4000. Even if you negotiate it down to a $1000, that's still a pretty big sum for most people. And your contact will be taken as an admission you owe, particularly if you agree you owe, and then the matter will be referred to a bill collector to hassle you to make you pay. Handling it yourself usually ends up with you paying about $1000 to make it all end.
Hire a lawyer to respond. That will cost $200-300, and usually ends the matter for you. Once Getty's lawyers know you have a lawyer, Getty's lawyers are ethically required to only contact you through your attorney. To avoid this, they once referred the matter to bill collectors (i.e. not lawyers) who would continue to contact you. However, that has not worked well for them as this is not a bill because there is no contract or court order saying you owe them anything. They have now dropped that stunt for fear of getting sanctioned for ethical violations. So a lawyer being involved means Getty then has to determine if it will sue or drop it. So far Getty has not sued to the best of our knowledge as generally the picture is already taken down and the contingent fee attorneys do not see the suit as making financial sense. Thus, for $200-$300 referring it to a lawyer basically ends the matter.
So, I recommend the third option. The 1st option is a reasonable alternative if you are good at ignoring demands without getting