yeah if the image is blocked then yeah, it won't work, but if they did read it more than likely they would click show images/html. But like I said it is a quick hack it is limited. The other service, use a more sophisticated method and it works for me, I know it email was opened. And if it was opened more than once, I know more than likely it was read.

I almost never view HTML emails, and if I do, I load pictures even more rarely.