One technique you might consider amounts to composing your mail in Microsoft
Word, which you probably have as part of Office. Set up a template in Word
with the logo you want, graphics and all. If all you want is a letterhead
sort of thing, that's all you need, although you can get fancy with fill-in
fields and so on. (To create a template, just create the logo part that will
be the same in all mail messages, and select Save As, then select Template
from the popup menu of file types. Word should switch the Save window to its
templates folder. Name the template as you wish.)
When you need a new message with the letterhead, open the Project Gallery
(Cmd-shift-P in all Office apps), and select the template there. It opens a
copy of the template in Word, where you compose your message, with whatever
text styling you wish.
When the message is ready to send, go to Word's File menu and select "Send
To->Mail Recipient as HTML". This will copy the Word document into an
outgoing Entourage mail message, reproducing it in HTML format. You cannot
edit the message once it is in Entourage; all you can do is address it and
send it. But this will give you the ability to send mail with a "logo", as
you request.
