Images must have a description, a.k.a alt text
Providing a text description for images on webpages is arguably the single most important step in making pages usable through screen readers.
Often the text alternative is called the image's "alt text", named after the attribute in HTML that contains the text. However, sometimes the text is called the image's description, because when inserting an image through OmniUpdate, the Description input box is used to provide the text alternative.
Authoring the alt text is not trivial; it must serve the same purpose as the image itself.
Rather than recreate examples and discussion, see the excellent Alternative Text from WebAIM, the first reference below.
- Alternative Text from WebAIM.