Dumb hack I did, googled around for pieces and got a nice-ish way to do this.

The change_desktop.sh file will work standalone, just change your
image path and have at it. show_desktop.py has the same functionality
as the "show desktop" button (in a helluva lot less code!) of Gnome,
showing the desktop for 2.0 seconds and then restoring the windows.
This is commented out in the shell script by default since it can be
annoying, for example if you want to watch something...

However, if you want to use this in a cronjob (as I did), you'll need
to make use of the included .make_Xdbus file which makes a .Xdbus
file in your home directory. All you have to do is
put it in your Startup Applications for Gnome, or run it when you log
on sometime. Cron has a different environment than the user, so if you
try to do graphical stuff on behalf of the user things can get a little
messy. The shell script needs the first two vars, the Python script
needs the last DISPLAY var.

Then, you need to make your crontab file look like this:

* * * * * . /home/yourname/.Xdbus; /path/to/change_desktop.sh

Don't forget the leading "."!

When I get bored I'll probably update this with a singular Python script
that supports multiple directories and so on.

I claim no copyright over this code.