“If I am missing information because the image is gone, then it means the image is informative and needs alt text.” - Caitlin Geier1
Informative Images
If an image provides information, it must have alt text. Does the image serve a purpose? Does it illustrate a part of the story not contained in the copy (or caption)?
Decorative Images
Some images are purely decorative and do not convey a message, provide value, or further the story. Decorative images have no additional value and do not add context to their surrounding content. For users of assistive technologies, describing these images may add unnecessary noise to the page. They should be made (effectively) invisible when there is no value.
Storytelling / Emotional Images
Images that exist to elicit an emotional response may be called emotion rich images2 or storytelling images. These may seem decorative.
In instances where the user can navigate past an image and skip the description, it may be appropriate to give this type of image alt text. In scenarios where the user can not skip by image fields, it is appropriate to leave them without alt text.
Alt Text Guidelines
• Do not start alt text with phrases like “Photo of”, "Image of", or “Illustration of”. VoiceOver will announce "Image" prefacing the alt text.
• Decorative images that do not convey content or meaning should be hidden from assistive technologies. The alt text field should be left empty so they do not announce "image".
• If the image is conveying something meaningful to the reader - use alt text to convey the meaning. You don't need to describe everything in the image or describe it in a decorative way.
• Include all critical information, such as numbers and other data.
• Do not duplicate information that appears in the image's caption or the content of the page. If the image itself is described elsewhere on the page, keep the alt text minimal.
• Energy.gov requires alt text to be at least 20 characters long3. This is a good requirement to keep in mind - if it's less than 20 characters, is it conveying the meaning of the image?
• Alt text should not be longer than 250 characters. Be concise.
1 https://www.deque.com/blog/great-alt-text-introduction/
2 https://tink.uk/text-descriptions-emotion-rich-images/
3 https://www.energy.gov/eere/communicationstandards/alt-text-requirements-web-images
Additional Resources:
https://www.w3.org/WAI/fundamentals/accessibility-intro/
https://www.boia.org/blog/should-you-include-alt-text-for-pictures-with-captions