If you select HTML as your default email format then you
can use the "Signature" feature found in Tools | Options
to create an HTML signature that includes a logo and
nicely formatted text.  

While you could add an image to the Rich Text Format, it
is likely that the image will not display correctly when
sent to a recipient over the Internet.  This is because
Outlook typically converts Rich Text Format messages to
Plain Text.  When that happens, your logo ends up being
an attachment to the message.

Hope this helps!

CW