Cell phone pictures are great if you can get a close up. Text the message to someone you trust then confront the guy in a very polite manner. There may be a logical explanation (I doubt it) especially if it is a new hunting spot to you. He could be lost, or has been hunting this spot for years and didn't know anyone really cared. At either way, don't get into a shout out or a shoot out. You've got the picture and you can decide what to do based on the quality of your conversation. If he gets loud... just walk off politely and call the GW.
A soft answer turns away wrath, But a harsh word stirs up anger.