Wallpaper image goes back to default after restart

On macOS High Sierra, your wallpaper resets to default after a restart because the system preference cache file is corrupted, or because the image file is stored in a location (like iCloud or the Apple Photos library) that isn't fully accessible or mounted before the login window loads.

Try this Move the image locally: Move your chosen wallpaper file directly into your local ~/Pictures folder instead of pulling it from iCloud or the Photos app.