You're using the wrong approach. Anything in the Resources section is not sent out with the mail, it is only referenced. That means that the recipient has to have the same resource if he is to see it.
In other words, only the people in the company can possibly see the logo if it is part of the design. I doubt that that is what you want.
I suggest creating a stationary with the logo embedded in it. In that way, the actual image will be sent along with the rest of the mail. Of course, that means that the stationary will be reserved for external communication, since you probably do not wish to clog your mail files with thousands of copies of your logo.
It also means that people will have to actively choose to send a mail with the stationary.
You could also create a default message. I believe that R5 and R6 can include a message defined by the administrator and included in every mail. Unfortunately, that message will be part of the footer, and not the header.
Pascal.