An interesting tidbit of code has shown up in the Chromium change log for Chrome OS. The change outlines that the default wallpaper on a device can be controlled by the master configuration. In other words, manufactures like Acer, Dell and HP could have specific wallpapers for specific Chrome OS devices.
The ability for an OEM to set a default wallpaper has been in play for some time now. Both Acer and HP have a custom wallpaper that they have all their Chromebooks show when you first power them on. Google is incredibly strict with what OEMs can do to the Chrome experience. To this point, the manufactures had one wallpaper they could add and it was the same on every Chromebook.