All photos, images and graphics-as-text must have equivalent alt tags.
Alternate text, also referred to as an "alt tag," is a label that describes the contents of a photo or image. Screen readers (programs that read Web pages aloud to blind and low vision users) read this alt tag to the user.
Without the alt tag, the blind web surfer will have no way of knowing what the contents of the image are. This alt tag needs to be added by the person building the Web page. An alt tag should be succinct; usually one sentence long.
If necessary, create long descriptions of your images using the longdesc attribute.
Sometimes one sentence in the form of an alt tag is not enough to convey the concepts of an image or photograph. An example would be a chart, graph, or technical drawing. In these cases, the long description attribute is useful.
A long description is a separate page that contains a paragraph or more of explanation. It can be as long or short as the web page designer wants it to be. The following outlines the general procedure for creating long descriptions:
<img src="../map.gif" alt="election map" longdesc="mapexplanation.htm">
With that said the longdesc attributes is generally not used. If the details of a complex image need to be explained to someone that is blind, most likely ALL users will benefit from this explanation. Therefore, it is usually a better choice to just include a text-description for all users to see that is next to the picture. Sited users can visually read it and a screen reader will, as well!
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