I think i've posted like 20 times my method of getting pics, i have got 8 in a normal way and bought a few..
Always give them a perfect foreign fruit when they want fruit
Always give them customised furniture, and look on the birthday guide
http://www.thonky.com/animal-crossing-new-leaf/villager-birthdays/ to see what furniture they like, or give them something thats fits their house
Give them gold stags if they want a bug..
Sharks if they want fish..
I usually do about 10 errands over the space of a few weeks
on the next errand, save before you hand over the gift, if they dont give you the pic, reset and try again, if they dont give you the pic within about 10 attempts, give up and do some more errands
Shout their name into the microphone to test for postitive friendship
If they send you letters daily without you sending them one, thats a good sign
if they come into your house interrupted
if their dialogue changes when they give you a gift, for instance freya used to say when she gave me something "i know we're not the very best of friends but can't i gift you?" this changed when she was close to handing her picture out..