At most, there are seven days where Callie could be communicating with her past self. There has to be a Prime Callie. The Callie we are following is not the Prime Callie. The Prime Callie would find the photos, see all of the ones of her and Jasper, and likely wonder if this guy was planning to blackmail her. She would not immediately tell the others, and she definitely did not know it was taking pictures of the future right away. She would have read the journal and investigated the pictures before leaving the room and become suspicious. She probably noticed the time on the clocks as well. How could she test the camera without Jasper and Finn knowing? So she knocks the coat rack over, then goes to check the photo and discovers that the new one is of tbe party the next night and that her photo from that day now shows the coat rack being knocked over. She changed yesterday’s future so it made the future photo change, too. This is the only explanation of why she thought the photos of messages had any ability to change anything. She wasn’t delusional, Finn and Jasper were just deceived about the time travel rules… by her… intentionally.

So then Prime Callie just has to take a photo at 8am the next day to tell her past self to knock over the coat rack and to hide the daytime photos in order for the previous day’s Callie to figure out right way that the photos are from the future. Notice that Prime Finn’s first painting is of a bunch of mail. Callie is his muse, so he gets inspired by seeing her get the guy’s mail. Once Prime Finn paints a painting, it shows up in the photos, but Prime Finn is not trying to reenact anything. He is genuinely inspired. Prime Callie may not have told Finn and Jasper about the future photos for several days to try to find ways to connect with Finn and guide her previous day’s self without him knowing, but the next day Callie tells them off the bat and Jasper immediately screws everything up by his bets.