The way I work is...
-I build a friendship with the villager before letting them move out
-When they start asking me random questions, I assume their friendship is high enough
-I let them go when they ping
-The mailtime after they move, I get their picture out of the letter
I'm assuming that's what you mean by the picture part.
If you mess up, don't worry. Just move 16 villagers in and out of your town and find them again "

then you will have a second chance.
- - - Post Merge - - -
Also, I doubt that's the point of the game. Where would you keep 333 pictures, anyway?