Rhetorical tropes employed by Barbara Kruger's, We don't need another hero:
Synecdoche- a stereotype of masculinity is portrayed by a boy flexing his muscle. Additionally a stereotype of femininity is communicating that all women are impressed by male strength and in dire need of being protected.
Antithesis- the text contradicts the message of the wholesome 1950's Rockwell painting. Kruger's design was made into a billboard during the late 80's when Reagan was president and militarism was a focus; the myth that fighting in a war increases masculinity, is being challenged.
parody- this is possibly a imitation of U.S. propaganda posters.
image source: http://lookintomyowl.com/barbara-kruger-pre-digital.html