Dock: remove icon inside the tooltip

[Camaleón (Gmail)]

Hello,

I'm trying to find a way to remove the icon that shows along with the
tooltip text when you put the mouse over the dock icons.

I found this documentation¹ link which apparently talks about this option
but after some unsuccessful tryouts in "~/.gtkrc-2.0" file I finally
opted to ask :-)

Is there a way to achieve this?

(I'm using Debian Wheezy with XFCE 4.8)

¹http://docs.xfce.org/xfce/xfce4-panel/launcher

Greetings,

-- 
Camaleón