The tracking images aren't very foolproof. Many mail clients (Outlook 2007, Gmail, probably more) don't load images in emails unless the user specifically allows it for that message or sender. You aren't likely to get any false positives (which may be why it could count as "proof of delivery") but it sure would be subject to plenty of false negatives. Tracking images have been used by spammers for years to verify which addresses are alive and which messages have been opened, so image blocking is increasingly standard.

If you want absolute verified delivery, you could also send via one of the services that promises secure emails by not actually transmitting any of the message in the email. The email is essentially just a link that opens a secure site from which the recipient can not copy and paste. Self destruct options can be set as well. You would be able to tell exactly if and when the message was viewed. I find these a little cumbersome, but it is the only absolutely verifiable method I can think of. It can not, however, promise the recipient was paying attention