@Falconet, this is magic: my icons had the default size, 48. I changed them to 40, as you've done, and now xfdesktop4 doesn't change their position anymore. Experimented logging out and also restarting, in both cases their position is not changed anymore.
I'm running Xubuntu 15.04 with xfdesktop4 4.12.0-1ubuntu1.
@Falconet, this is magic: my icons had the default size, 48. I changed them to 40, as you've done, and now xfdesktop4 doesn't change their position anymore. Experimented logging out and also restarting, in both cases their position is not changed anymore.
I'm running Xubuntu 15.04 with xfdesktop4 4.12.0-1ubuntu1.