Anything in /opt/zimbra/mailboxd/webapps/zimbra should be accessible on just
http://server.domain.com/file.png
(If your not using multiple domains/virtual hosts: Instead of the zimbraSkinLogo[LoginBanner/AppBanner] you could also replace files in /opt/zimbra/mailboxd/webapps/zimbra/skins/_base/logos to cover most skin's.)