Zimbra
- Version 7.2: not possible.
Thunderbird
Edit > Preferences > Composition > Send Options: 'Send the Message in both, plain text and HTML'.
Edit > Account Settings > <account> > Composition > Compose messages in HTML format: On.
Edit > Account Settings > <account> >
- Signature Text: Use HTML - ON
In the signature: <img src="http://www.math.uzh.ch/..../signature.png"> or <img src="file:///home/a/user/signature.png">
- Deposit the image on the personal homepage.