A thumbnail is a reduced version of an image which can be clicked on to show a larger version of the image.

This is commonly seen in online shopping web sites where a small photograph of a product is dsplayed on the prodcuct information page and the visitor has the option to inspect a larger, more detailed picture by clicking on the thumbnail. It is also common to see online picutre galleries laid out as rows of thumbnail photographs, each of which can be clicked to reveal the full size photograph.

When you use graphics or photography on a web page you should include Alt Text for each of these pictures. Alt Text is contained in an Alt Tag and is text that describes the picture so that visually impaired people using screen readers can have a description of the picture read out to them. The Alt Text is not displayed unless the picture is missing or the visitor runs their mouse over the picture.

For a site to be standards compliant each picture should have and Alt Tag, though it is not necessary to add Alt Text to each picture.

An example Alt Ext would be "Acme 1000 Widget. Click this picture for product details"