Thumbnails are reduced-size versions of pictures, 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, Mac OS X, KDE (Linux) and GNOME (Linux).

Some web designers produce thumbnails with HTML coding that makes the user's browser shrink the picture, rather than use a smaller copy of the image. In principle the display size of an image in pixels should always correspond to its actual size, in part because one purpose of a thumbnail image on a web page is to reduce download time. The visual quality of browser resizing is also usually less than ideal.

Displaying a significant part of the picture instead of the full frame can allow using a smaller thumbnail while maintaining recognizability. For example, when thumbnailing a full-body portrait of a person, it may be better to show the face slightly reduced than an indistinct figure. This has the disadvantage that it misleads viewers about what the image contains, so it is less well suited for searching or a catalogue than for artistic presentations.