This really stretches the meaning of propaganda almost to the point where it's synonymous with individual speech. Propaganda is systematic opinion making where some group does things like call tons of people or distribute tons of leaflets, or astroturf internet forums. It's not the same thing as putting up a mural on a building you own, or a sign in your yard.

I think a mural could be part of a propaganda campaign, but I don't think by themselves they merit the categorization.