For inserting one logo in one signature I would just edit the signature
and copy/paste the logo (or use the advanced/external Editor (Word) and
insert the image there).

If you were referring to inserting the same logo for all signatures
across the company then the following project is worth taking a look at:
http://sourceforge.net/projects/gensignfromldap

Basically it generates signatures for each user (e.g. from a logon
script) based on Active Directory data and a layout template (which
supports images).

Dave wrote:
How do I insert a company logo into a signature for Outlook 2000?