I don't let my mail clients display remote content by default, and if I were to receive an email where the actual body was just a remote image, I don't think I'd show the image.
I don't understand the reasons for someone to use a service like this
Not all email clients display images by default, and I could easily see this being bypassed by caching the images.
Someone care to elaborate on technology? Obviously it must be sticking some remote content in there somehow eh?