Someone's been photobombing me - what gives?

So Prompto takes pictures all day long. Usually it's your characters, some scenery, monsters you fought that day, etc.

Recently, someone who definitely wasn't there when the photo has been taken has been showing up. The game has even specifically pointed this out to me by having the characters mention it. I'm on chapter 9 and this seems to happen about once every few days... is there any significance to this?

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.}