Graphics such as illustrations, icons, photos, screenshots, and thumbnails all require text equivalents for use with assistive technologies. Each text equivalent should briefly identify the graphic and, if appropriate, describe any functionality that the image represents. The text equivalent must be brief, descriptive, and accurate. You need to include as much information as possible into the fewest number of characters without sacrificing intelligibility
A text equivalent should describe the visual appearance of the graphic. The text equivalent can also include instructions on how to take an action, such as how to select options in a dialog. Ideally, a text equivalent provides the user with the same information as the original graphic. To find out how to implement text equivalents in your documentation, see the http://developer.gnome.org/projects/gdp/handbook.html.
To comply with Part 1194.22 in Section 508 of the Rehabilitation Act: Electronic and Information Technology Accessibility Standards, documentation must include a text equivalent for each graphic in documentation.
Follow these guidelines when you write text equivalents: