There is a video on getting villagers to move out & some of it had the friendship levels and how it worked - not sure how accurate it is, but it actually seems pretty accurate and the tips in the video did help me in getting rid of some... unwanted villagers, to say the least.
Anywho, video here:
http://www.youtube.com/watch?v=6jIuEm8Bq9I
From what I read though, you're in a good place once they visit you without telling you, gifting you stuff through the mail, and so on so fourth. I think once you do an important task for them like... signing a petition for them they
may give you their picture, but I think it's kinda random. Also if they've moved out and they were really close to you, they'll include their picture in the mail. But again, doesn't mean the only way to get it is for them to move out.
There's that Blanca mini-game as well and I heard you can get their pictures from that too.
From the Animal Crossing wiki: "Blanca appears again in Animal Crossing: New Leaf as the event character for April Fools Day.[1] Unlike past games, she will go to a villager's home and pretends to be that villager. The player is then left to decide which one is the original and which is the copy. If guessing correctly, they will receive a picture of the villager as a gift. If the player guesses all the villagers correctly, the player will receive Blanca's picture."
Sorry for the wall of text, LOL. Just trying to help you out! c:
Also I've had this game for like maybe 3 weeks or a month now and I haven't gotten a single picture so I'm pretty sure it takes quite a while or you gotta get super lucky!
