Send out one of the images with a unique id per mailout, this could be the
users email address if you wanted to tie in the data per user (may be
illegal in some countries without users permission).
I don't know if it's illegal, but if I would be a subscriber I wouldn't like it if someone is colleting information without informing me or asking my permission. Perhaps you're intention isn't to collect every piece of information you can lay your hands on, but thats something your subscribers can't verify.
If it's clearly stated on the page where people are subscribing you might get less subscribers.
Bye,
B.