I'm connected through SSH. In other words, I have no mouse nor keyboard connected to the unit. How do I change the wallpaper from the terminal?
You can set the Raspberry Pi desktop background with this command under Pixel (lightdm) Desktop Manager:
From http://lxlinux.com (section 5 on the page):
In a traditional set-up wallpaper can be changed by terminal launching a wallpaper GUI: pcmanfm --desktop-pref, or by directly editing the .conf files in ~/.config/pcmanfm/LXDE/. The default wallpaper folders depend upon the distribution and are generally /usr/share/backgrounds or /usr/share/lxde/wallpapers.
Sounds like you need to look at the .conf files in the ~/.config/pcmanfm/LXDE/ folder.