The official wallpaper is here:
I assume one can get the official image from this tar:
Of course we cannot use that color, but our banner should incorporate
the beaver in some way along with our plasma wallpaper or colors. That
file is available here:
You can see our present banner on the website: http://kubuntu.org/ .
It needs to use that same font face for the Kubuntu part, as that is
official. We had to decline to use quite a few lovely submissions last
time because the font was changed. The Plasma wallpaper doesn't need
to be blurred as it is in the 17.10 version, but that is an option.
We'll use this on Facebook and Twitter as well as the website.
Posted on ML: https://lists.ubuntu.com/archives/kubuntu-devel/2018-April/011629.html (and kubuntu-users)