Thumbnails are a previsualization tool that allow for quick iterations of shot ideas. They are quick, hand-drawn sketches and each panel is usually not much bigger than a person’s thumb — hence the name. The level of detail is purposefully low to allow for ideas to flow quickly. This also helps you psychologically when it comes time to throw out bad ideas, because you don’t feel like you’re throwing away something precious.
In most cases, the primary focus of a thumbnail panel is on the characters and the shot size. Backgrounds are less important. Their purpose is to sketch out a conceptual plan for the visual progression of the shots.
Here are a few examples of what thumbnails typically look like…
As a point of contrast, compare these with more polished storyboards, such as these ones from the movie Rango, where there’s a much higher level of detail in the characters, the backgrounds, the tonal values, and sense of space: