[X4U] Change Desktop Picture at custom interval

[Eddie Hargreaves]

I used to have my Desktop picture change every 3 or 4 hours using the tip
provided at http://www.macosxhints.com/article.php?story=20030325020900756

Unfortunately, in Tiger, I can no longer open com.apple.desktop.plist in
TextEdit and change the values. How can I change the values using defaults
in Terminal? 

I know it needs to start with:
defaults write com.apple.desktop

But I don't know how to format the rest of it.

-- 
Eddie Hargreaves