Well this is taboo a bit...

March 1, 2005 by Chris Charlton

In some parts of the Internet world this is not considered a trusted part of email and newsletters. Some advertisers append the email address or ID to an image, say the header graphic, and they track all the requests for that capaigns images and see what email where read.

Wonderful email programs like Thunderbird automatically block image requests for privacy concerns, and have 1-button to click and show images for the email/ad/newsletter. So don't trust the mentioned technique since it's already 2005.