Wallch is a nice wallpaper changer application for Linux. You can automatically change your desktop wallpaper from your picture collections, a Live website, Wikipedia Picture of the day or even a Live Earth wallpaper. The latest version of Wallch 4.1.5 comes with a better user interface. You can simply click on the wallpaper source on the top bar.
To install Wallch 4.1.5 on Xubuntu 14.04, use the following commands:
sudo apt-get install wallch
Alternatively, you can download Wallch 4.1.5 DEB file from Sourceforge link.