Well it's an utterly useless method of measuring how many people opened the mails you sent out, but it'd probably be reasonably easy to implement.
You could just make the address of the image something like
Then it's a simple matter of parsing the server logs. Like I, and Felgall, have already said, though, it's an utterly useless and completely inaccurate way of measuring how many mails were opened. Not that an idea being utterly useless has ever stopped companies from trying to sell it to you in the hopes that you won't realize how utterly useless it actually is.