Whatever its words, a poster without a striking image is a missed opportunity, and incongruous, vaguely disturbing images often work best. (The snake is among the most unsettling creatures on earth to gaze at, yet it figures as the sympathetic subject in not one but two great American political images, the “Don’t Tread on Me” Gadsden flag and Ben Franklin’s “Join or Die.”) For World Press Freedom Day last year, a journalists’-advocacy group in Jordan came up with this simple design. Yes, today’s tyrants are more interested in clamping controls on keyboards, blogs, and cellphone transmissions, but for evocativeness it’s hard to beat the chained nib of an old-style fountain pen, trembling somewhat as if in resistance.

Today, social media and meme culture endlessly rework classic posters and poster genres for purposes of commentary and satire. That stands in a great tradition: as a means of persuasion, posters are themselves a powerful part of the press. Use them in a good cause, and enjoy them too. [Earlier entries in this series: Monday, Tuesday, Wednesday, Thursday]