Question: Surly Duff tells a kid to "Take a picture, it'll last longer" - just what does this expression mean?
Answer: Along with the other answer, it's a sarcastic thing that people say to someone who they don't want to be friends with or date. ie. a group of "cool" kids not letting a "loser" type into their group, or a woman rejecting a guy who wants to date her. They are basically saying "I want to get away from you as fast as possible, so you might as well take a picture - it'll last longer than our friendship/relationship would."
Answer: It's just a sarcastic response people say when they catch someone staring at them. As if the person staring is trying to burn an image in their mind of what that person looks like and a picture will last longer than the memory.
Bishop73