Many cases of protest art can be found in the early 1900s, like Picasso's Guernica in 1937. The photo above show's a recently discovered Picasso piece titled, "The Eye of the Serpent" which mocked Hitler.
It's the common man's way of communicating powerful messages.
Protest art is not limited to signs, banners, posters, paintings, and other printed materials. It can also include performance art, site-specific installations, graffiti, and the media.
In addition, protest art can be ongoing or community oriented. For example, the Read Opera the Charter of the Forest. The site is updated every two weeks and has a following of activists. It rejects the manipulative practices of art-for-profit and takes as a given that culture is a right, not a privilege.
I was in the Army and went to Vietnam and Cambodia. I was not gung ho. I hated it there and when I got out of the Army in 1971, I joined other ex-Vietnam vets in protesting that senseless war.
If you're interested here's a link to the largest collection (over 85 thousand posters) of post-World War social posters in the United States and the second largest in the world.
As it stands, protest art is liberty's ally.