Just providing an image button and its functionality in the html editor toolbar to accept url's of the image has been completed via
Bug 27017 - Need Insert Image toolbar button in Signature Editor (see attached pics) Which makes the feature visible and more intuitive for the user to add an image.
You can also do things like:
zmprov ga
user@domain.tld | grep zimbraPrefMailSignatureHTML
zmprov ma
user@domain.tld zimbraPrefMailSignatureHTML "<html whatever></sig>"
Exposing it a little more is:
Bug 27398 - Access to HTML source of signatures from web-ui
Later this image functionality may be extended to allow image attachments which
is this bug we're in.
Bug 25797 - RFE: inline/embeded logos in the signature
