Use the task scheduler and create a VBscript to change the wallpaper. Create a script for each wallpaper you intend to use.
dim shell
Set shell = WScript.CreateObject("WScript.Shell")
wallpaper = "C:\path\to\wallpaper.jpg"
shell.RegWrite "HKCU\Control Panel\Desktop\Wallpaper", wallpaper
shell.Run "%windir%\System32\RUNDLL32.EXE user32.dll,UpdatePerUserSystemParameters", 1, True
Save the file as something.vbs and add it to the task scheduler, and voila! You got it all working.