Take parchment and ink, copy a name from your memorized names list. Anyone with this parchment in inventory will have the name of that character pop up as if he had him memorized too. The name that pops up is the same as the one memorized by the poster maker.
Maybe have an exploration vs stealth check for recognizing a face from a poster.
Another exploration vs stealth check for comparing two posters to determine if it's the same guy.