I discovered this one accidentally. In Apple's Terminal.app, under the Terminal menu, choose Window Settings..., then select Color from the pop-up menu. Under Background Settings, you can set the background to be an image -- but you can also choose a folder instead of a particular image.
Set the destination to a folder full of images (and clicking Use Settings as Default) will cause each new Terminal window to use a different random image from the chosen folder. It's these "just works" features that make the Mac so cool.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20070713170839728