Desktoptopia is described as 'desktop background manager for the Mac that automatically loads and displays designer desktops on your monitor, as often as you wish'. There are three alternatives to Desktoptopia for Windows. The best alternative is Wallpaper Juggler, which is both free and Open Source. Other great apps like Desktoptopia are WallManager (Free) and Walyk Wallpaper Changer (Free).
Wallpaper Juggler is a configurable app for Windows 7, Vista and XP to download wallpapers from InterfaceLIFT.com or WallpaperStock.com and change them periodically. Features It runs in background and can be controlled from System Tray.
Walyk Wallpaper Changer allow you to change your wallpapers automatically or manually. You can load files (jpg/jpeg, bmp or png format) or entire folders. You can also configure hotkeys and monitor the chantes made in a specific folder.