Here is a program I created some time ago. You will need to have zenity and feh installed in order to use it. The program also assumes that you start your system with startx or a dm like Slim, which uses .xinitrc file to startup desktop and programs alike.
Below you will see a list of screenshots, which will clarify how to use the program.
Copy the program to /usr/share and run it within a terminal using the command above. You can change the location path but then you also need to tweak some lines within the program (this is easily achieved).
This is the main window. Do note that the program might have some functionality problems with (some but not all) gtk3 themes/engines. For best functionality gtk2 themes/engines are recommended.
Setup window. If you run the latest PostX Gnu/Linux you can skip this step. This step is also only required to run once.
Setting the wallpaper. First message.
Setting the wallpaper. Selection time.
And finally, here is the program located inside the zip file.