Related link: http://www.technewsworld.com/story/34289.html
In an article posted today, Alastair Rampell tells Tech News World:
If you’re upset that your friend sent you an e- mail using DidTheyReadIt, then that’s a problem between you and your friend.
If he really thought that, why does DidTheyReadIt have to use a web bug? In most cases, I do not think people are even going to know that someone has used the service.
I know that someone loaded the web bug in the message I sent to his email address (alex@rampellsoft.com), and apparently had it open for over four minutes from something in California. I have not received a reply yet, but I know the message got there.
And, not only that, I posted one of their web bug URLs (http://didtheyreadit.com/index.php/worker?code=844eea38c4f0ab9bd2220f65f4107dbe) in my use.perl journal and it has been picked up by a couple of spiders. I am not sure why spiders are loading images, but I guess some do.
Image spiders
I imagine that Google's spiders populate both the text and image indexes.
Image spiders
and the Yahoo spiders rip the images and put them in their image database complete with search terms based on text near the image in the original page.