The figures ("pictures") aren't part of the proof, they're part of the presentation, and Figure 6, as stated in the paper, is included as a reference to an analogous work.
If you read the last 1/4 of the paper, you'll see that Figure 6 is an integral part of the discussion; furthermore, nothing in the discussion is proven rigorously --- they all appeal to the picture.
That is exactly what a proof by picture is.