Macworld: Mac 911: Auto-changing iPhoto 6 desktop pictures: If you run into this issue, there are a couple of easy workarounds to rotate iPhoto pics.First, use iPhoto, select an album, then hit the Desktop button in iPhoto. This will open System Prefs, allowing you to randomize your iPhoto selection. However, this does not seem to work for mulitple monitors (only the main), and the selection is not auto-updating. You'll have to repeat the process if you add other photos you want rotated.
The other way is a little more complicated and requires some .plist hacking. See the link above. Using this method, multiple monitors should be able to use the same photos.