Given that they're expanding to reddit, I was wondering how dbzer0 is setup to handle similar requests. The legal help page for dbzer0 is just an e-mail address.
As soon as that image is loaded, the remote http server knows the IP address of the client viewing the image.
I bet that it does in private messages too, though I haven't tested it. Send a private message to a user, referencing an image on a server you control -- maybe even a one-pixel, transparent image, a tactic that has been used in Web tracking in the past -- and the server knows their IP when the image is viewed. Even if it doesn't, you could probably just respond to a few comments by a user in regular threads, and they're probably going to be the first to view the image (and probably the only to view all of them).