How to write useful alt text

Alternative (alt) text is a textual description of images on webpages. Non-text elements like photos, graphs, figures, and animated GIFs can increase reader engagement and support understanding of the material. Alt text provides a brief description of functional images to those using screen readers. Decorative images do not present important content and do not require alt text descriptions. 

When the content of an image is presented in adjacent text, adding alt text can be redundant. However, such an image may link to more content and require alt text due to the navigation function. Confused? This WebAIM article on Alternative Text offers examples of functional and decorative images and appropriate alt attributes.