Web Accessibility Policy of the State of Maine
4.1 - Provide "alternate text" for all images.
The HTML image element (<img>) includes an "alternate text" attribute (alt) that is used to provide text that can be substituted when the image itself cannot be displayed. Alternate text is meant to be a concise replacement for an image and should serve the same purpose and convey the same meaning.
Why: Individuals who are blind cannot perceive information presented in images; screen reading software reads alternate text instead.
ALL images must have appropriate alternate text. As a rule of thumb, consider what you might say if you were reading the web page to someone over the telephone. Alternate text should be brief, no more than a few words (150 characters).
Certain types of information, such as GIS and geographically coded data, currently may not be available in a displayable text format. At this time it is acceptable to use these formats without a text equivalent. However, these formats should be used with caution and only when necessary. If a more accessible format is available, or becomes available, to present the same information it should be used instead or provided as an alternative.
Specifically: For images that contain words or letters - use alternate text that includes the same words or letters. For images links - use alternate text that identifies the link's destination or function. You do not need to include the words "link to." For images that are invisible, purely decorative, or otherwise do not convey meaning - use alt="" (null) to indicate that the image can be safely ignored by a screen reader.
Ref: WCAG 1.1; 508 a
4.2 - Provide full descriptions for graphs, diagrams, and other meaningful images.
"Meaningful" images are images that convey more information than can appropriately be expressed as alternate text.
A full description allows a user who cannot see or understand a meaningful image to receive the same information as a sighted individual.
Present a full description of a meaningful image either on the page on which the image appears or through a link immediately preceding or following the image. Use alternate text to provide a concise name for the image. For example, the alternate text of a graph should state its title and the long description should summarize its trends and/or present a table of its data.
Note: The longdesc attribute of the <img> element can also be used to provide a link to a full description. Because longdesc it is not yet supported by most web browsers, it should not be used as the only method of providing a full description.
Note: Certain types of information, such as GIS or georgraphically coded data may not be currently available in a displayable text format.
Ref: WCAG 1.1; 508 a