If you see an inappropriate photo on Foursquare, you can flag it in the app and select one of the following reasons:
Hate or Violence
Unrelated (Android only for now)
In the iPhone app, tap through to the inappropriate photo, then tap “Flag” on the bottom right and select a reason. In the Android app, tap through to the inappropriate photo, tap 'Report Photo' on the bottom left and select a reason. The BlackBerry will have photo flagging implemented in the next update.
If you are unable to flag an inappropriate photo in the app, you can report it by tweeting our support Twitter account @4sqSupport.