Esetroot (you'll need to install eterm for this) is what I use.
Just add the following to .xinitrc
Code:
Esetroot -scale /path/to/wallpaper.jpg &
You can also add the following to your fluxbox menu to give you the opportunity to choose wallpapers from the menu.
Code:
[submenu](wallpaper){}
[wallpapers](/your/wallpaper/directory){Esetroot -scale}
[end]
-geomatt