Jamf has a guide here if you'd like to lock the Desktop Wallpaper down:

https://docs.jamf.com/best-practice-workflows/jamf-pro/deploying-wallpaper/Deploying_Wallpaper_to_Computers.html

There's a slightly different method if you want to allow users to change it

https://community.jamf.com/t5/jamf-pro/help-setting-a-desktop-background-image/td-p/292564

In short:

1. Push a package that contains the image you want to use
   1. Unless you want to use a stock image
2. Push a configuration profile with the Restrictions Payload
   1. You will find the wallpaper under the Functionality tab

Thank you all for sharing valuable information. You saved my day :)