desktop backgrounds??

[Beartooth]

	This has got to be a FAQ, and I suppose I'll kick 
myself when I read the answer. 

        I'm running Fedora 20, Mageia 4, and CentOS 6.5, on 
three different PCs, all behind a 4-port KVM switch. (The
choice of machine for the 4th port is waiting on an issue not 
relevant here.)
  
        I have a collection of pix that I fancy for desktop 
backgrounds, aka wallpaper, with copies on all machines.
  
        I use the different backgrounds in much the way I 
use the various color-coded tabs in my gnome-terminal. 
They tell me at a glance where I am, without requiring 
the distraction of conscious thought.
  
        CentOS is still enough like Gnome 2 so that I can put up,
and occasionally change, the background(s) for it. In Fedora and
Mageia, I use Xfce 4; but I have been beating my brains in vain 
against menu after menu without success.

        The main menu in xfce has a "Desktop" choice, with which
I seem to get *almost* to control; but at the very end, no place
I can find to click (right, left, or middle) on seems to set the 
pic. What am I missing??

-- 
Beartooth Staffwright, Neo-Redneck Not Quite Clueless Power User
Remember I have precious (very precious!) little idea where up is.