Very nice project. Here are some ideas for premium features:

-Change monitor. If images stop matching, issue an alert.

-More locations in general. Obvious additional server costs.

-Screenshots by user agent; iOS, Android, IE, Firefox, Chrome.

-Screenshots by other variables. Is a particular ISP injecting ads? etc.

Change monitor/alerts would be useful for detecting problems traditional monitor tools may miss. Additionally may be useful in contractual situations where another party is employing cloaking. E.g. I bought a static button banner and the seller is switching it out on me.