If you pining away for those gob-smacking Windows 7 theme wallpapers, here is how to install them in Ubuntu
2.Install p7zip-full via Synaptic
3. Change the .theme extension to .7z and open with archive manager by right-clicking on the file
4. Extract the wallpapers and install via Appearance Preferences
If you really need to get the wallpaper to change automatically every so often, be a devil and install Wallpaper Tray.
While you at it, you may as well change the Xsplash background picture.
Installing Xsplash-Background-Settings will allow you to choose one of your new wallpapers as the background. No more gloomy Ubuntu.
Add Xsplash-Background-Settings ppa
deb http://ppa.launchpad.net/meerkat/stable/ubuntu karmic main
apt-get install xsplash-background-settings
The programe will ask you to paste a piece of code in a terminal to change the permissions on the xsplash folder. Do it.