When I see that someone has a graphic in their email signature, I try
to encourage them to ditch it. It always implies that each of their
email messages has an attachment, and sometimes I search for or sort
by messages with attachments.
It's not what email is best at, and nobody in the world will think
your organization less professional if you don't have your logo in
your signature. And there are ways of formatting your signature so it
will look good.