How do I uninstall Wallper?
Quit the app and drag it from Applications to the Trash. macOS restores your default wallpaper automatically.
To completely remove Wallper from your Mac, quit the app from the menu bar, then drag Wallper.app from the Applications folder to the Trash and empty the Trash. macOS will restore the previous system wallpaper without further action.
If you want to remove cached wallpapers as well, delete the folder ~/Library/Application Support/Wallper. Settings live in ~/Library/Preferences/app.wallper.plist and can be deleted the same way.