Ning has added yet another new feature to Ning 3.0! A fellow Creator found the feature, before an announcement. Kudos to ThisIsBully for his awesome detective work.

• Edit your Photo Module, or add a new Photo Module to any page.
• Select Display: Slideshow
• Select Transition: Slide or Fade
• Show each photo for: 3, 5, 10, or 20 seconds.
• Update and Save.

See screencast for demonstration.

Note: It may not be ready for prime-time. But, it worked for me.