Final Fantasy 15 – What Constitutes a Side Quest?

That is your custom marker, you can place those on your map by hitting square over a spot on the map. Those are just used so you can see on your minimap which direction you need to go to get to your destination.

Based on your comments, the "person" in the pictures is actually Gentiana, the Messenger of the Gods. She does it occasionally and someone points it out if you look at the picture on the reel.

There is no benefit to this aside from getting the Immortal Photobomb trophy/achievement. After that, it's just for fun. (Personally, I'm saving every picture she manages to get in)

There is no significance other than the fact that it's showing that she is with you, despite you not knowing it.

_{Note: her wikia article does include spoilers unrelated to this lower down on the page, so read at your own caution.}