I'm going with my Pony Express stamp. I don't have a scan of it, so just this photo. Reason: It's the Pony Express! This stamp rode on a horse's back during that short but famous period of mail delivery.
Not the exact stamp, but the memorable design of one of the stamps included in the starter packet that came with my kid's album. An exotic image from an exotic place that helped start me down the rabbit hole of stamp collecting.