When Anna gives Lucas half of the photo booth pictures he gets the one of them talking and the one of them kissing. Later on we see Anna in her room she is looking at the same two pictures but Lucas is supposed to have them.
This isn't true. Lucas and Anna kissed twice, the second and third pictures. Anna tore the strip after the second picture, so each of them has a picture of them kissing. Lucas has the photos of them talking and the first kiss. Anna has the photos of the second kiss and the one for bonus points, which is covered up.