Although a bit cliche, I like using an image of a red stage curtain about to be opened. They give a sense of anticipation, look beautiful (nice warm colors, lots of detail), but at the same time focus all the attention on you, the speaker, since there is not that much to look at at the projector. I got the image below from iStockPhoto (referral program link), many other stock photography sites have many, many of them.

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