What constitutes good alt text?
All major document publishing formats support alternate text for images, often abbreviated "alt text". The purpose of alt text is to provide a text equivalent of the image, so people who are unable to see the image have access to the message it’s intended to convey.
What constitutes good alt text for an image depends on the purpose of the image. The World Wide Consortium has created a tutorial that provides a variety of examples for images that are informative, decorative, or functional, as well as images of text, complex images, groups of images, and image maps. For specific instructions, see the W3C Web Accessibility Tutorial on Images.
For additional information regarding alt text you may wish to consult the following related Knowledge Base articles: Are there guidelines for describing complex images?, What is the current recommendation for providing long descriptions for complex graphics?, and How long can an "alt" attribute be?