Just three so far. I have Twiggy, Avery, and Cherry. I never even actively tried to get Cherry's. In fact I barely even talk to her. Just one day she asked me to get her a fish so I did and VOILA, got her picture.
I've gotten around 10 (Marshal, Whitney, Leonardo, Pashmina, Fauna, Melba, Amelia, Tutu, Dotty, and Colton). Either via doing a task for them, in a letter or when they visit my house o-o
I only keep two pictures in my house -- Puddles and Paula. Those two are the only ones that I got doubles of. I used to store all my villager pics in my house, but I started getting a lot, so... I made them into a museum exhibit instead.
I have none ; _ ; but I haven't really concentrated on getting any from villagers, I'm atm trying to get ALL my dreamies and then after that I'm gonna decorate my town with paths/trees/bushes/pwps. etc. Then try for pics.
Katie's pic is one of the things she can send you after you take her to another town. I do think it's kinda rare, though, I only ever got it once, most of the time, I just get sketchbooks and cameras and digital photo frames, haha.