alex2011 What does the journal say before that? Which step are you at, exactly? The "confront Peter / not yet" part after finding the photo? IIRC after a few days pass you can trigger an event with Susan instead.
The thing to keep in mind when trying to trigger an event in this game is that most events, once triggered, go on a cooldown (main quests - so you can continue; random events - so they can be repeated) - a "you have attempted this recently, please wait before trying again" of sorts. I think this is what happens once you find the picture: the Susan part has a delay before it can be triggered, but then you simply need to encounter her and the quest goes on. Confronting Peter at this stage achieves nothing.
Either way, with all of that in mind AND assuming I guessed the step you are stuck at correctly, the next step is: ignore Peter, run into Susan at school, the quest is going to progress automatically without the need to go to your office and call Susan there (doing so does nothing, anyway - IIRC all parts involving Susan are "randomly encounter her at school", rather than "call her to your office").