Whether Anna become Pure or Slut is entirely depends on what kinds of dialogues you chose when you're interacting with her.
If you respond Anna with loving, supportive dialogues like how a good boy should be, she will act more and more loving towards you ( the Pure Path ).
If you respond Anna with naughty, bold dialogues that constantly pushing her to embrace her sexual desires, she will act more and more like a slut you want her to be ( the Slut Path ).
Taking pictures of Anna is purely for progressing the Main Storyline regardless which Route you want her to be, but I believe you can find different kinds of photos based on which one of Routes you chose.
If you're wondering if whether you can change Anna from Pure to Slut or other way around, you can do it when she only has 1 or 2 Hearts. Once she has 3 Hearts or more, I believe you can't change her Route ever again for the remaining of the game, since she has different dialogues for each Route itself as well.