Is there a way to change the icon used by the Quit or Lock Screen applets in the panel?
First I found the plugin and had a look at the conf file.
the actions.desktop is the plugin you want.
If you have a look into it, see the following entry:
Starting a "find" for it will result in this:
The xfce-system-lock.png is the other symbol you may want to change.
Xubuntu 6.10 "Edgy Eft" Xfce 4.4 beta 2 with ATI Radeon Mobility 9600