The difference between a thumbnail and a sketch is that the thumbnail is bound by the orientation and proportions outlined by the project or art director.
It is a small, thumbnail size-ish drawing that tells two things: the basic composition and the basic concept of the image. A good art director will tell you, “I want three compositions for this concept,” or “I want three concepts for this scenario.” That should prompt you to draw one concept three ways, or draw three concepts one way.
This here's a thumbnail for a potential image for the 2017 book of short stories titled "The Man Who Shot Out My Eye Is Dead." by Chanelle Benz. I like one-eye stories and folks whose names end in "Z", so even though I haven't read the book, I can speculate.(Eye joke!)
Specs: The image is squarish - or so the thumbnail indicates. Someone is standing in front of a shooting range. They appear to have a shot out, or, at least, a very dark eye. He or she is holding an urn. Draw conclusions.
Facts: The flower on the urn is a stinking corpse flower! It grows the largest individual bloom in the world, larger than 3 children, and smells like a rotting body! The shape of the shot-out eye person is similar to the target silhouettes. The shot out eye and the stinking corpse flowers shapes are also similar. Alice Walker, who I used as my hair model for this piece, is a one-eyed wonder. This comparing and contrasting of shapes and relationships is also a strategy for making strong or stronger concepts (see concept vs. idea) and visual communication.
Fun things: Give the AD something to worry about so they don’t harp about your drawing skills or lackadaisical visual strategizing: Why does he/she wear a Frenchy-like striped shirt? What does that have to do with anything? What is the subject a he or a she? Are all shooting ranges found in the desert?