"Alt text (alternative text) describes the appearance or function of an image on a page. Alt text is read aloud by screen readers used by visually impaired users, displays in place of an image if it fails to load and is indexed by search engine bots to better understand the content of your page." [from - What is alt text?]
- Use accurate and equivalent descriptions.
- Describe the image thoroughly, but succinctly. The text should describe the content and function of the image.
- Include any text contained in the image.
- Do not use the phrases "image of..." or "graphic of..." since screen readers note it is an image.
- Decorative images do not require long descriptions and should be "nulled" or left untagged as an image.
From Butler University LibGuide