Sophie, there are actually two possible endings for this twist:
1. If the person in the photo is innocent, in the end you'll be told that the murderer planned to kill him/her next. (You mentioned that)
2. If the person in the photo is guilty, you'll be told that he/she took the photo of himself/herself immdeiately after the murder.
And I think I had cases when the second person killed was not the one in the photo. So I believe this twist does not have to do anything with game mechanics - it is purely of entertainment.