/usr/share/wallpapers/SoftWaves is in desktop-base 9.0.2+deb9u1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/share/wallpapers/../desktop-base/softwaves-theme/wallpaper
/usr/share/wallpapers/../desktop-base/softwaves-theme/wallpaper