Free-design editor — click on canvas to add text, upload images, drag, resize, then copy HTML.
These point at the shared images from assets-admin.php. Replace one from there (e.g. swap the promo banner for a new offer) and every signature built with these buttons updates automatically - nobody has to touch their own signature.
Tip: For Gmail/Outlook — paste into signature settings. Images you add are compressed and uploaded to the server, then linked by URL rather than embedded, so central updates reach every signature.