If it's a unique image, then presumably Google reads it and pre-caches the image on receipt. So all you'll get from 1 pixel image is confirmation of email deliv

If it's a unique image, then presumably Google reads it and pre-caches the image on receipt. So all you'll get from 1 pixel image is confirmation of email delivery, which is something you get with SMTP anyway. Of course if Google wait to cache the image when you actually read it, then this isn't true. But I'm prepared to give them more credit than that.