my suggestion was to try to change /usr/share/pixmaps/openbox.png, but it seems you tried variations of that already?
it says
here that you should look in
/usr/share/icons/<current-theme>
/usr/share/app-install/
/usr/share/app-install/icons/
/usr/share/pixmaps
(however i'm not sure what to look for - it could be some sort of "unknown-application" or "default" icon.
my personal experience is that icon themes vary hugely in which icons they offer.)<=not ob-specific
i think openbox (and that would be only the window decorations, and openbox' own menus, which you can't even access on a default lubuntu install) falls back to that default icon only if it cannot find anything matching, and i think there's nothing you can do about it, i.e. its practically hardcoded.
the only thing you can do is to always provide openbox with an icon.
- either by checking each applications .desktop file and
- - either change its icon to one that exists in /usr/share/pixmaps or your current theme
- - or add an icon of that same name to /usr/share/pixmaps
- or use a utility like xseticon, please see
here.