Background Program

In the KDE Control Panel  one of the options for "Background" is
"Background program.  This intrigues me, as I have a perl script that
automatically downloads the Astronomy Picture of the Day from NASA every
morning, and would like to have it be able to set the background at the
same time.  However, I have been unable to find any documentation on
background programs.  The big question I would like answered is how
should a background program operate?  Should it return a reference to an
image on the filesystem that should be made the background, or is there
some library call it should make to set the background, or is there some
other mode of operation?  I have not been able to find this documented
anywhere.

Thanks,

Andy