This is an animated version of a lovely old Victorian puzzle. Basically, you have to count the number of people in the image, wait for the piece to flip around, and count them again. The flip should happen after about 10 seconds (if it doesn’t, you might have to click on the image and have it open in a separate window)
My thanks to @DrBrocktagon for bringing it to my attention. 10 points to anyone who can explain why it works!