As you may know, Wallch is an open source wallpaper changer similar to Variety.
Among others, it has support for Gnome 3, Unity, XFCE, LXDE and Mate, it rotates the wallpapers you place in a folder, can be configured to set the Picture of the day from wikipedia directly on your desktop, has support for displaying a certain site on the desktop, this getting updated automatically at a given time interval, and last but not least, it has support for Wallpaper Clocks, a feature that displays a beautiful clock over your wallpaper.
The latest version available is Wallch 4.187, which has been recently released, coming only with small fixes.
Up to date packages are available via some third party PPA, so installing the software on Ubuntu 15.10 Wily Werewolf, Ubuntu 15.04 Vivid Vervet, Ubuntu 14.04 Trusty Tahr, Linux Mint 17.x and other Ubuntu derivative systems should not cause too many problems.
All you need to do is add the ppa to your system, update the local repository index and install the wallch package:
$ sudo add-apt-repository ppa:noobslab/apps
$ sudo apt-get update
$ sudo apt-get install wallch
Optional, to remove wallch, do:
$ sudo apt-get remove wallch