If this guy is always on a VPN, running an "invisible" image off a hosted site to get the IP connection won't work. But, if they happen to be a little careless, then depending on the way you can contact him you could glean something from this.

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Honestly, though, I don't have high hopes when it comes to these ransom attempts. If you give in, they will ask for more: the leverage is on their end and only increases every time they are paid. If this guy has these images, then they may as well already be in the public domain.