You can add wallpapers from your home folder. If you don't want any of the system included wallpapers, remove the installed mint-backgrounds-* packages. Those install wallpapers under /usr/share/backgrounds/, but that is a system folder and I strongly recommend you do not remove or add files there. Remove the installed mint-backgrounds-* packages if you don't like the system wallpapers, and use wallpapers from your home folder if you want extra.
Linux Mint 14 Nadia / 64-bit / Cinnamon