B737 First Officer reports his aircraft being taken out of service by Maintenance due to a picture being forwarded to the FAA by a passenger showing a possible anomaly with the right engine.

Narrative

We had an unscheduled aircraft swap in the morning and we flew the aircraft two legs. I properly preflighted the aircraft and found no discrepancies. All the logbook paperwork was in order and the aircraft was airworthy. We flew; uneventfully; two legs and on arrival; Maintenance met us at the aircraft. At this point; we were unaware of any maintenance issue; and we did not call for them at any point. Maintenance notified us that; per the Company; an inspection was to be done on the aircraft; on the spot. Apparently; the previous day a Passenger took a picture of the right engine at the slat. This picture was forwarded to the FAA. Once we were aware of what was going on; the Captain immediately notified the Chief Pilot's office. They requested we comply with Maintenance and their decision to ground the aircraft. We deplaned and continued on with a different aircraft without further incident.A potential maintenance issue on the aircraft disclosed to the FAA. NOT visible on ANY walk around. It is virtually impossible to prevent this type of occurrence in this day and age. Cameras exist in every form.