Once upon a time, I knew...but no more. Are you running DSL? It has a menu in the control panel to configure the background (I always use that, which is why I forgot how to do it manually -- about the only time I run Flux anymore).
Found this, though... http://www.newlinuxuser.com/howto-set-background-wallpaper-in-fluxbox/
Just installed Zenwalk on an old laptop today. Pretty sweet...433mhz and 198 mb of RAM, still boots up in under a minute. XFce has come a long way since the first time I ever used it.
Gonna get the wireless working tomorrow, mainly because the driver disk is upstairs and I'm lazy.
Though it'd be kinda cool if I could also use it for KDE. :)