Phones are now very personal items.
Long time ago in order to perform an effect with pictures; you would have to carry the pictures with you.
But now people carry pictures with them all the time. Million of pictures!!
Using your spectator's phone and his Instagram account the effect goes like this:
Your spectator selects FREELY any category of pictures. He searches for this category himself; then your spectator selects any picture he wants. This picture could have been uploaded by anybody in the whole world! Yet you read your spectator's mind and accurately find the username that uploaded that picture.
Totally impromptu, start clean, end clean.
This is USERNAME