Nitrogen is a: “X11 background previewer and setter”
You may use nitrogen to preview and set a wallpaper in Gnome, KDE, fluxbox or openbox, with KDE or Gnome is really simple to use it, just open it in the appropriate directory, the one which contains your pictures, then choose one and make it your wallpaper.
But to use it with Fluxbox, you can start the same as with Gnome or KDE, but each time you restart Fluxbox it will not appear as your Wallpaper.
So, to make fluxbox reload the wallpaper each time you restart it, edit your fluxbox init file.
and add this line at the end of the file.
session.screen0.rootCommand: nitrogen --restore
Now, you can change your wallpaper just using nitrogen as explained above, and you will have it loaded each time you start fluxbox.
I think is better than, Using feh to set fluxbox wallpaper, because you can easily change it, and no need to edit the overlay file.