To start Fluxbox with a wallpaper, you will have to edit some configuration files, and have a image viewer installed on you machine, not all of them works, so consider using one of these two.
Esetroot or feh. To install Esetroot run:
sudo aptitude install eterm
You should see something like this:
feh is a nice wallpapersetter. You won't have any problems.
Now lets edit your
./fluxbox/init file with your favorite text editor (mine is vim with colors), and add this line if it is not present.
Now create a file
~/.fluxbox/overlay and inside it put this lines:
background: aspect background.pixmap: /path/to/your/file.jpg
Now just restart fluxbox and you are done.