F7 : gconf cache : how to invalidate it?

[Skunk Worx <skunkworx verizon net>]

I use gconftool-2 in a few scripts to change the desktop background image.

In FC6 I used :

gconftool-2 -s -t string  /gnome/desktop/background/picture_filename 
/tmp/a.jpg

In F7 this works, however if I overwrite /tmp/a.jpg with a new image, 
and re-run the gconftoool-2 command, the image doesn't update.

I've tried changing to another name (e.g; nonexistent or existent) and 
then back to /tmp/a.jpg, also unset/set of the picture_filename file, 
but the old a.jpg is always ends up on the BG even though eog, gimp, 
etc. show the image has changed.

Could someone tell me how to invalidate the cache and force a re-read of 
the image?

TIA,
John