Xfce System Wide default backdrop

[Yves-Alexis Perez]

Hmmh, as far as I can tell, there is no system-wide setting for
xfdesktop default background. It's #define'd in
common/xfdesktop-common.h and then a desktop.xml is created in
$HOME/.config/xfce4/mcs_settings/desktop.xml when xfdesktop4 is first
run.

Did I miss something, or the only way to change the default backdrop is
to do it at build-time ?

Regards,
-- 
Yves-Alexis