Installation:

Copy the folder day_of_ubuntu into /usr/share/backgrounds. (Alas, the system here does not seem to do relative paths!)
Now, in appearance settings, add day_of_ubuntu.xml as a wallpaper. (Drag and drop is the fun way!)


Legal stuff:

The attached wallpapers are by various authors, and released under various licenses.
I have linked to their respective web pages and attached license texts for good measure.

Dawn of Ubuntu (dawn.jpg) and Ubuntu Spring (noon.jpg):
Created by Armin Ronacher
Creative Commons Attribution 2.0 Austria License (see http://creativecommons.org/licenses/by/2.0/at/)
http://lucumr.pocoo.org/art/

Night of Ubuntu (dusk.jpg):
Created by Roberto Della Ragione (based on Dawn of Ubuntu)
GPL License (see file gpl.txt)
http://www.gnome-look.org/content/show.php/Night+Of+Ubuntu?content=52958

Midnight of Ubuntu (midnight.jpg)
By Dylan McCall (direct modification of Ubuntu Spring)
Creative Commons Attribution 2.0 Austria License (see http://creativecommons.org/licenses/by/2.0/at/)


As for day_of_ubuntu.xml, do whatever you want with it. Really. Sell it for all I care.


Thanks,
Dylan McCall
<launchpad.net/~dylanmccall>
