problem setting background image_filename

[Patrick Jacquot <patrick jacquotf free fr>]

Hello list!

I'm currently on Fedora core 6 (GNOME 2.15.3)
or Fedora core 7 (GNOME 2.18.3)

I wrote a little script who generates images,
in .jpg form.

That script writes the images alternately on 
~/temp2.jpg and ~/temp2bis.jpg.

After having written a new image, the script
sets /desktop/gnome/background/image_filename
to the filename where the image has been written.

On GNOME 2.18.3,The desktop background changes actually,
but the images displayed are always the two first
images generated.

Could anybody help me to fix that, in other words,
what did I wrong?

(on one machine with GNOME 2.16.3, another version of the script
works fine, even whitout changing the filenames, i.e. all images are
written on ~/temp2.jpg. On another (faster) machine with GNOME 2.16.3
and the version of the script who doesn't change the filename, 
the only image displayed is the first generated.)
even