I believe it is theoretically possible to embed a virus into an image file that can run arbitrary code on someone's computer. This is probably the only way you will be able to get more than the creep's IP address on your own. How this is done is outside of my area of expertise, and I would caution that sending a file with a virus is almost certainly illegal.
It's practical. There's a science around it called steganography which embeds arbitrary bytes into an image, and the quality reduction of it is minimal and barely noticeable.
Yes I know. Steganography can also be used to hide messages 'in plain sight', somewhat similar to the timestamps on videos by al qaeda and the Taliban in the early 2000's.
The "virus"
Any code that does something without the consent of the end user is malware, and is likely illegal to be sending to others. Gathering information about a computer without the consent of the user, I believe crosses the threshold to sending malware.
Even if you were able to identify the person behind the blackmail, I don't think there is much the girl or Eddie can do. If you tell the person you know his identity, he can destroy evidence of his crimes, and possibly disseminate the pictures you want to keep private.