The principle of thumbnails as we are using them in class is to simplify a painting or a photo or something from life down to a singular aspect: line, shape/pattern, depth (using line), and tone. This is to help us deconstruct an image so we can use it in our own paintings. In the example below, it can also help us to try to uncover some of the decisions the artist made about, e.g. composition, light, shape, pattern. See below for an example. First is the original painting then there are the four thumbnail "deconstructions":