Reviews are tied to app store accounts, which are tied to credit cards. Reviews also reset with each version of the app. You could detect the same accounts being used to review each version of the app, discount ratings from newly created accounts, discount ratings from accounts with reused credit cards, etc. It seems like an easier way to game the system would be to pay anyone with an iPhone $5 to download and rate your app.
You can have an app store account without a credit card. I don't think it was always possible, but it is now. Also, the accounts don't have to be newly created. This operation could be quite long-lived.
You could have some automated system which loads many different apps to be rated, so that the person in the picture isn't rating the same app over and over again in a short period of time.
