Mr. Brainwash -- real name Thierry Guetta -- tells TMZ, RHCP hired him to use street art to promote their new album ... set for release in August ... which is why each poster contains the date 8/30/11.
Brainwash -- who starred in the infamous Banksy documentary "Exit Through the Gift Shop" -- wouldn't specify how much he's getting paid for the gig, but tells us, his job is far from done ... in fact, he and RHCP are teaming up on several more projects.
We called the Peppers' peeps for comment several times -- but so far, no word back.
