From what I can gather these are photos of the uyoku trucks that travel around Japan broadcasting right-wing nationalist propaganda (or, “inspirational thoughts,” depending on what side of the dialectic you are on).
Some could also be bus ads for books, but that doesn’t seem to be the case.
If you can read Japanese, please confirm, correct or expand on my assumptions.
(Those characters beside Mishima are fairly clear. I would love to know what they say.)
This Flickr user posted a set of the right-wing trucks (some with Mishima, some without) and gave a first-hand account of the experience.