Letterhead is part of the design. To change, all you need to do is find the Subform that corresponds to the one you want to change, and modify it.
You must however know that the design is common to all Notes mail users, and that the image itself is not sent in the mail, only the subform name is.
In other words, if you send a mail internally, your change will not show up. If you create a new subform, no one else will have it and they will not see it. Finally, you'll also be changing the image for mail you recieve using that subform, which might not be what the sender intended.
Only if you send over the Internet (using proper parameters) will your version be sent.
Finally, you obviously need the Designer client to make the changes, as well as permission from your hierarchy if such modifications are against company policy.
Pascal.