Apple

That must be a new one in Leopard. In Tiger, you can get the name of the
current picture like this:

  set pFile to (path to preferences as Unicode text) &
"com.apple.desktop.plist"

  tell application "System Events"
   get value of property list item "LastName" of property list item 1 of
property list item "Background" of property list file pFile
  end tell

Yes. I'm on Leopard but your script works fine here as well - Even with the desktop image set to change randomly.

If I cause the desktop image to change using AppleScript (another Leopard feature) the script you posted will get the name of the image that was last set automatically by the operating system, not the current image. (if you set it you can't get it)

Waiting until the system automatically advances the desktop image restores the ability of the script to return a correct answer.

Joe