@Marilyn6I totally agree: I would prefer to have a pic of the actual guest. If you're not comfortable with it, I would ask them to upload a proper profile photo of themselves. If they decline, you can decide what to do after that. If it's an instant booking, you could cancel penalty free, citing that you're not comfortable with the guest.
One of ways this happens is that Airbnb sometimes pulls information from the guest's social media profile. So it may be their pic on Facebook and it just gets dragged over to Airbnb. It may not be the guest being deliberately trying to hide something.