I believe that, when I tried Openbox, the graphical program that I used to
change the wallpaper (and that usually comes pre-installed in Openbox
distros) was one called "Nitrogen".
But, I know there are others:
https://help.ubuntu.com/community/Openbox#Wallpaper
If you search on the Internet, there are some good pages about the Openbox
WM, that list all the programs that you should, by default, install alongside
with Openbox, if you plan on using it as a substitute for a DE.
And, if you try some live Openbox distros, you can also learn a great deal
from them, in terms of configurations.
