poster n.

"bill, placard, thing posted," 1838, from post (v.1). Poster boy/girl/child "someone given prominence in certain causes" is attested by 1990, in reference to fund-raising drives for charities associated with disability, featuring child sufferers, a feature since 1930s.

Earlier it meant "one who travels post" (c. 1600); "a post-horse" (1797). Sense of "one who posts bills" is by 1864.