This new script is a new design that has been tested to work properly on macOS 10.13 High Sierra through macOS 12 Monterey. In MDS, you set it up as a Script to be run “After macOS and packages are installed” and be sure to include your custom desktop pic in a Resources folder within the same folder as the script.
This script works a little differently than the old method so you’ll see the macOS default desktop picture for just a moment on first login before it gets changed to your custom picture.