It is as simple as Ivan is Trevor's guilt, manifested into a human hallucination.
He uses Ivan's supposed mysteriousness to start blaming his distractions and his recent obsession over who is out to get him when really Ivan does not exist at all.
Fight Club spoilers explain this best:
In a way it is like the ending of Fight Club where The Narrator turns out to be Tyler Durden. In the end, the picture that he kept claiming was Ivan, was really him.
The picture is taken before his accident and shows how Trevor was before the guilt of killing a child started seeping into him and causing him to lose sleep for a year. Notice that Ivan is finally gone from his life when he finally turns himself in for what he has done and is also able to finally sleep.