The ThumbsPlus image file manager showing folder tree in the upper left and 12 thumbnail-size images to the right.

Thumbnails are reduced-size versions of pictures, used to make it easier to scan and recognize them, serving the same role for images as a normal text index does for words. Visual search engines and image-organizing programs normally use them, as can some modern operating systems or desktop environments, such as Windows XP, KDE, and GNOME.

Note that while automatic thumbnailers reduce large pictures to a small size, the result may not be a quality thumbnail.

Some inexperienced web designers produce thumbnails by simply reducing the dimensions of a large image using HTML coding, rather than using a smaller copy of the image. In practice 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 has also sometimes been less than ideal.

Reducing a significant part of the picture instead of the full frame can allow the use of a smaller thumbnail while maintaining recognizability. For example, when thumbnailing a full-body portrait of a person, it is may 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.

In 2003, the court in the US case Kelly v. Arriba Soft Corporation ruled that in it was fair use for Internet search engines to use thumbnail images to help web users to find what they were looking for.

See also

• Contact sheet
• Thumbshot
• Image.
• Wikipedia:Extended image syntax (how to use thumbnails in Wikipedia pages).