Following on my signature post here, I decided to take the signature to the next level a little, and update the "how to" some. First off, Custom Attributes are supported for signature fields. I tested successfully seen here:
And, after some poking, I think all of the macro's are based on the Powershell attributes on the get-mailbox cmdlet. So run a get-mailbox USER fl and you can work from there. If you have a binary image attached in AD, I wonder if you could add that to a signature.