We are moving our mailserver to Zimbra and we're testing/evaluating its features.
One of the tests I've done is to enable the html signature and insert an embedded image in it (it is in the briefcase).
Now, when I send an email, Zimbra webmail interface shows always the email with attachments (i.e. the image that I attached to my signature).
Other clients (thunderbird for example) don't show the signature images as email attachments.
This is a convenient feature because it allows to distinguish emails with "true" attachments from "false" attachments (those in the signature).