Hey aot,
1. DMCA Takedown Notice is the way to go in the future. Get registered: https://www.copyright.gov/dmca-directory/
I have done this now, very easy. However, as it was a contractor of mine that wrote the article I'm not sure that this would give me safe harbor protection as DvG pointed out.
2. For now, how much are they asking you? Try to negotiate and bring the extortion price down. This step will also buy you some time to prepare your arguments whether in the court on through emails to Pixsy et. al.
They are asking 750USD, however, with the license that Pixsy offers it is very limited so I would not think its even work the $200 you paid, however, I would settle if need be.
3. Clean all of your sites and social media platforms. Use this time to both learn from your naivety and make yourself extortion-proof in the future.
I'm on it. Double checking that every image is CC and has proper attribution to the author.
4. Wait...and if you can give time to this matter then fight it till the end. The end is either they will stop harassing (emailing) you, or they go to the court and they'll lose, or you lose and have to pay according to the court's decision.
I'm going to try to keep this out of the courts, but I'm not going to be a walkover either.
Interestingly enough, the last email request from Pixsy had an attachment for ALL of the cases that Pixsy had on behalf of Marco Verch. Including who the license was issued to, the image that was licensed, and the fee charged. According to this, he has collected >$300K worth of licensing settlements with Pixsy! Including providing multiple licenses for the image he is pursuing me for.
I'm going to presume that this was a mistake of the case manager because giving me this information gives me some leverage against Marco & Pixsy. Presumably, they have client confidentiality agreements in place that giving me this information would violate.