This isn't so much a plea for help as I've worked this out myself now, but I just wanted to mention that it seems very hard to find out how to change the default Zimbra logos for those with the Network Edition.
The release notes tell you to go to the wiki, and there's nothing there of help - just information for the free edition.
Then, if you do enough searching you find several references to the rebranding document on the Support Portal. Maybe I can't see the forest for the trees, but I can't find it anywhere. I looked in the release notes, the admin guide, the other documents section. I searched for 'rebranding', 'logos', 'AppBanner', etc. And nothing of use was found.
Perhaps the release notes should be adjusted to point to a specific document.
And for the benefit of others, if someone far brighter than me can find exactly where this information is on the Support Portal they can explain it in this thread too.