Ok easiest way-ignoring the url pointing to zimbra.com EDIT: (because it has to stay that way-read the
licenses-the 'zimbra inside logo' should actually be zimbra.com/powered if you can)
go to /opt/zimbra/tomcat/webapps/zimbra/skins/_base
by modifying the _base folder you should simplify it to one or two steps so that you don't have to insert into every skin
make new images for the LoginBanner.png & the AppBanner.png that contain your logo & the
zimbra powered logo
keep them roughly the same pixel size (login banner's like 150px) transparent etc
see this to understand what I'm talking about
:
Customizing Themes and Adding Zimbra Powered Logo - Open Source Edition - ZimbraWiki
(the lite client pulls the images from another location-hunt around the tomcat folder for folders named img images logo etc)