Where does Linux Mint keep the files for default desktop wallpapers?

I suppose your Linux Mint comes with GNOME and normally, it should be under /usr/share/backgrounds .

Mint change background in each release, could get background of previous release.

Command - install background:

• sudo apt-get install mint-backgrounds-<release_name>, install a specific release,
• sudo apt-get install mint-backgrounds-*, all release,