Verified answer

Well, for me the obvious answer always in animation is that he just draws/paints them. The multiple pattern repeats behind the man holding the "balloon" can be done with clone brush of one worked out or by using template brushes - which I have never done more than play with. The flashing boxes are just a new image put up for one or two frames, then a different one, then back to the first, then remove it. This is pretty basic animation stuff, I would have thought - animation is tedious attention to detail.