these are "web bugs". some web pages have gifs that are 1pix x 1pix, and the gif alpha makes them transparent even, so they are absolutely invisible.

The bugs usually are more like http://www.malserver.com/webbug1.gif?1CDD5A62

The ? is a separator and passes the key following it, so they can look you up and see that you opened the page, or the email. The web server can at that time log your ip address even.

So they could send you an email with a webbug in it, and when you opened it, they'd (1) know you opened and read it, and (2) would have your ip address at the time you read it. (your WAN address)

Authorities use webbugs in email to track down people.

If a spammer includes such a tagged image in a spam, they can track the effectiveness of their spamming because they can tell which addresses actually got someone to open the email. ("we got a LIVE one here, add them to the Gold List")

I work for the Department of Redundancy Department