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, and GNOME.
Some 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 is also usually 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 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.
In 2002, the court in the US case Kelly v. Arriba Soft Corporation ruled that it was fair use for Internet search engines to use thumbnail images to help web users to find what they were looking for.
- The Denver Public Library Digitization and Cataloguing Program produces thumbnails that are 160 pixels in the long dimension
- The California Digital Library Guidelines for Digital Images recommend 150-200 pixels for each dimension
- Picture Australia requires thumbnails to be 150 pixels in the long dimension
- The International Dunhuang Project Standards for Digitization and Image Management specifies 96 pixels at 72 dpi
- DeviantArt, one of the largest artists community, automatically produces thumbnails that are maximum 150 pixels in the long dimension.
- Flickr, one of the largest photographers community, automatically produces thumbnails that are maximum 240 pixels in the long dimension, or smaller 75x75 pixels.
- Picasa, one of the largest photographers community, automatically produces thumbnails that are maximum 144 pixels in the long dimension, or 160x160 pixels album thumbnails.
The term vignette is sometimes used to describe an image that is smaller than the original, larger than a thumbnail, but no more than 250 pixels in the long dimension.
Art directors and graphic designers use the term "thumbnail sketch" to describe a small drawing on paper (usually part of a group) used to explore multiple ideas quickly. Thumbnail sketches are similar to doodles, but may include as much detail as a small sketch.