How can you get someones picture on animal crossing wild world?

First, you have to make good friends with them. Talk with them frequently, but not too much, do chores for them, don't hit them with any thing, don't trap them, and send them nice letters with gifts. After a while, when you walk by them, they should have a symbol pop up above their head, run after you, and when you talk to them, they'll give you their picture.