Small images placed to mark or decorate. A sticker carries feeling where words would be heavier.
Middle English stikken (to pierce, to fasten by piercing), from Proto-Germanic *stikan; the small decorated image is named for the plain fact of adhering, not for anything it depicts.
A small image placed to mark or to decorate, carrying feeling where words would land heavier. It sticks because that is the whole of what the word ever meant.