Hi,
The backgrounds in our distributions, depending on which distributions, will always be located somewhere under /usr/share/. This is the first point.
The second point is that you may want to also find where the configuration which points to the background is located. The background, in Openbox branded distributions, as far as I know, can be displayed by PCManFM or by FEH, or by the tool having for name Nitrogen.
In the remixes which I provide, it is either PCManFM (most of the time), or FEH (it is setup but not active by default).
The subdirectories where to look are always the same: either "wallpapers", or "background", or sometimes you can find it in a more specific location such as "/usr/share/lubuntu/backgrounds", or "/usr/share/xfce/backgrounds/".
PCManFM has a handy gui configuration tool, which is available in the main Openbox menu in Bento: Right-click menu on the desktop, > Preferences > Desktop preferences.
You can also launch it from console with the command "pcmanfm --desktop-pref".
To see all options available, you can type "pcmanfm --help-all".
If you want to see the configuration for feh, you will find it as command lines in the file $HOME/.config/openbox/autostart. There you will mainly find symlinks. They allow switching from English to French or to German easily, thanks to a script added and an entry in the main Openbox menu.
In Archlinux I am testing PCManFM 1.2 beta, and I just realized I don't know how to do that in pcmanfm anymore. o_o
I must consult the dev.
If you have more questions, I'll be around!
