I stand corrected there is a tree there. I was looking at Mytton's gif too much and missed the stem! However, the illusion still is great when you compare sizes of the people relative to each other and where they are standing. No one seems to know anything about the man and
? that are standing at curbside next to the President's limousine when the neck shot comes in. In Willis slide,he looks very small. There is a tall man standing there in the Bronson Film and someone else right beside him. I tried to blow up a picture to look at it in more detail. Unfortunately, you need an original to look at. It appears that whoever had there head next to him, suffered an "erase" out in the Bronson pictures I have been able to find on the internet.
As well, there is something alongside of him, that appears to look like a dog (object?). It certainly can't be a child because no hand is restraining them from running out - the man in the black suit clearly has his back to that object and standing on the curb.