In Time Lapse, Callie was hiding the morning photos. She is secretly sending herself messages. What is the use of that? Why is she doing that? Then she kills her own boyfriend! What is Callie trying to do?
She discovered that the machine was printing 2 photos per day. She was hiding the morning photo from the guys so that she could send herself messages on how to change/prevent events from happening. He goal is to use the machine to change the past to save her relationship with Finn.
The problem at the end is that the note she was leaving herself fell off the window which sealed her fate and ended it all. He wasn't supposed to die, but she knew she could leave herself a note to prevent it from happening, but the security guard came along and ruined it.
Edit for comment:
When it's discovered by Finn that there were two photos each day and Callie was hiding one of them, she tried to explain. Finn decides he is going to destroy the machine, so Callie shoots him to prevent it from happening. Her reasoning is that she can leave herself a note to not get caught (i.e. not let the hidden photos get discovered) which will avoid Finn wanting to destroy the machine and thus, Callie not having to kill him.
Edit: I found this link that explains everything in detail TIME LAPSE