I would not include an actual image for a logo as that would then swamp your Exchange server as EVREY message going out would include it. Then as people reply or forward you have it again.

I would use a link to an external source (which means e-mails would need to be set to go out as html)
Don't forget though that there are many places that will strip it to text format anyways and loose the link or logo.
 
The information is stored in the registry for the signature though I think.