A new Zimlet is released that adds a button in the compose field in Settings->Signatures which allows the user to add images in email signatures.
You can download the Image in E-mail Zimlet via the Zimlet Gallery and then deploy it as usual with
zmzimletctl deploy zimbra-zimlet-signature-image.zip
I deployed this one but it broke some of the admin zimlets we use in zcs 8.8.15, such as com_zimbra_viewmail
Modern UI Zimlets are for Zimbra 9 and above.
ah ok thanks that explains it