I hope this message finds you well, present circumstances excluded. I am a licensed IP attorney with over a dozen years of experience handling matters of this nature. It is a pleasure to assist you today.
First off, understand that this is a cottage industry....that is, threatening suit for digital infringement of copyrights or supposed copyrights. The fact of the matter is that while, in a strict sense, this is a copyright infringement, it lacks intent. Intent is what equates to money in the infringement world. Moreover, a copyright suit needs to be able to show intent in order to receive attorney fees. Since that is not present here, it is HIGHLY unlikely that they will file suit against you under the circumstances for one image which was promptly removed. They are bluffing in all likelihood. They are professional blufferes.
That being said, you need to send them a certified letter with a return receipt (legal notice and due diligence which is good evidence for you if litigation is started) in which you ask them to provide to you a copyright of the actual copyright for the particular image they allege you have infringed. Ask them to provide it to you within 10 business days of the post mark on your letter.
In many cases, just this tactic alone will cause these trolls to back away - never to be heard from again. It forces them to show their hand in a way. If they do respond with a copy of their actual copyright, then you may offer then $500 as a nuisance value to settle. Nothing more. That will insulate you from chances of being sued and that is not admissible against you later should they sue (which they are not).
If they will not or cannot produce the copyright, I would just ignore then after sending one more certified letter in which you point out your doubts about their copyright and in which you state that even if it were a copyrighted image, any use you may or may not have utilized was removed the moment you received notice. There was no intent to use this alleged copyright.
Then, you ignore any other copyrights. While they can sue, it cost way more than they stand to gain to sue you. It is just a poor business decision, so they will not in the end.
The letters are there just in case as an insurance policy to show your due diligence here.
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Thank you and best wishes moving forward!