I don't use fluxbox but if it uses the PCMan file manager you may be able to change your background image there.
Go to edit > preferences > Desktop There you can make your wallpaper choice.
I've seen this work in lxde and icewm. let me know how it goes.
Ok so I downloaded fluxbox with gslapt, changed my desktop manager to kdm so I could choose it as a session and I'm now using fluxbox, and yes the background image is just an ugly yellow-gold. I see why you want to change it.
So, I gave my above suggestion a try.
I used the 'Run' utility from the menu and entered pcmanfm.
It gives a warning that the GTK+ icon them is not set...just ignore it and click OK.
I go to edit > preferences > Desktop
There I select all 3 checkboxes so that pcmanfm manages the desktop and shows the fluxbox menu on right click.
I browse and choose my background picture from /usr/share/wallpapers
Click ok and just like that I have a new background (much better)
pcman remembers the background choice has to be started to do so. So here is the trick to getting it to work everytime.
Go to /.fluxbox it is a hidden file in your user directory.
There there is a text file called startup. Open it with mousepad or the like.
There is a section in it that looks like this...
# Applications you want to run with fluxbox.
# MAKE SURE THAT APPS THAT KEEP RUNNING HAVE AN ''&'' AT THE END.
# unclutter -idle 2 &
# wmnd &
# wmsmixer -w &
# idesk &
pcmanfm & (<---add this line here and save. It will start pcmanfm when you start fluxbox.)
Now you can easily change your background and it will reappear each time.
Now to get rid of that pesky warning.
Open .gtkrc-2.0 (this is another hidden text file in your user directory.
add this line to the top of it and save.
That should do it.