I myself have my wallpapers stored over a wireless network and if I lost connection when my wallpaper was supposed to switch I had many problems. Introducing Network Wallpaper Switch, a program that is a lot more versatile and won't cause problems if windows can't find your wallpaper files. Most wallpaper switchers or cyclers out there will give errors or remove files when they try and switch to a file that is unavailable.
This causes a lot of issues especially if you are accessing the wallpaper files over a wireless network etc. So with this wallpaper switch program, if a wallpaper is unavailable it will just wait the interval and try again on a different wallpaper. If you do want to remove files from the list that aren't available anymore there is a Verify Files option. This version is the first release on CNET Download.com.