The image is framed using the brass knobs for the elastics along the rear edge,
and the white beads along the front edge.
I sight along these features as shown above.
The blue line corresponds to the center of the image,
The purple lines show the extent of the left pinhole image,
and the red lines show the extent of the right pinhole image.
I also use beads on the left side panel of the camera to find the vertical extent of the image.