While the Desktop and Screen Saver preference pane allows you to choose a set of pictures from iPhoto as a set of rotating desktop images, it only allows you to select an entire album.
To select just a few pictures, highlight them all in iPhoto using Cmd or Shift, then click the "Desktop" button. This adds them to your Desktop and Screen Saver settings as a new folder called "iPhoto Selection," where you can change timings as usual.
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