Thumbnails are quick and rough drawings. They rarely have any real amount of detail. The purpose of the thumbnails are to visualize concepts and layouts quickly and make it more visually effective and dynamic.
Rough thumbnail sketches for the story mentioned above.
The important thing to keep in mind while thumbnailing is on the emphasis of strong shapes supporting composition without going into the details.
Once they are approved they are further developed into conceptual pieces.
The thumbnails are further developed into final inked layouts.