Can I make a window pseudo transparent on X11 with xprop?

Question

To change a window transparency I can set the opacity property:

xprop  -f _NET_WM_WINDOW_OPACITY 32c  -set _NET_WM_WINDOW_OPACITY 0x7fffffff

But instead of a "real" transparency that overlaps other windows I just need to see a desktop picture as a background. Is there any property that I can set with xprop to achieve this?

I am looking for a solution for xfwm window manager.