I have an idea of changing wallpaper as specific time to fit to themes. Like at the work hours: The wallpaper will be my todo list or "Don't be stupid, Do your work" wallpaper, at exercise time it will be a cool body to remind me.
I just quick google and I found Automatic Wallpaper Changer but I'm looking for a lightweight app. If you know one of them, post it in your answer :)
OS: Windows