The answer's a bit more obscure, but I worked it out a while back. You need a profile picture to be able to message someone on here, so they just use a sample photo of a koala (amongst others) that is included in Windows rather than search elsewhere online for a pic, or put their own photo up. I've seen a few people use the other sample photos of tulips/desert/etc here and there on here. Why the koala one is the most popular for them to put up is anyone's guess.
9 years