Changing panel background? (impossible?)

[pazuzu legba pwrtc com (Jon Craig)]

How do you change the panel background graphic independent of a theme?
 
If I change themes, the panel background changes to the theme-defined one,
but if I either drag a graphic from the file manager (as suggested in the
FAQ) or use the right-click "Background Settings" menu, I get "I've detected
another panel already running, start a new one as well? (The new panel will
not be restarted)".  If I answer Yes I get the same panel as before, if I
answer No the panel vanishes altogether.  This seems to be a bit more
trouble than it should be just to change a background!  What am I missing?

This is a stock install of GNOME/Enlightenment right off the RedHat 6.0 CDs.