Quick Answer: A thumbnail is a reduced-size version of a picture or video, used to help in recognizing and organizing them, serving the same role for images as a normal text index does for words. In the age of digital images, visual search engines and image-organizing programs normally use thumbnails, as do most modern operating systems or desktop environments, such as Microsoft Windows, macOS, KDE and GNOME. On web pages, they also avoid the need to download larger files unnecessarily.