Set custom background dynamically by copying image to `/usr/share/backgrounds` using a script

[Chaitanyabsprip]

ln -sf $(find /home/chaitanya/Pictures/ -type f \( -name "*.png" -or -name "*.jpg" -or -name "*.jpeg" \) | shuf -n1) "/home/chaitanya/.wallpaper"
# sudo cp $(readlink $HOME/.wallpaper) /usr/share/backgrounds/mirage.png
nitrogen --set-scaled $HOME/.wallpaper

I am using the above script to change the wallpaper of my wm.
I would like to dynamically change the greeter background by copying the symlinked image to /usr/share/background
currently doing this results in a gray background.