I think my DD was that same age when she picked up a book on the JFK assassination, flipped to the photos, and asked pointed questions about what they depicted. I had the same reservations you have, but I went ahead and answered her questions.

No obvious harm was done, and it might be better to introduce this sooner rather than later. Most pre-schoolers don't seem to get the concept of permanence, and already being familiar seems to soften the blow for later.