Good alt text tells a reader who cannot see an image what matters about it in that context. Describe the information or function the image contributes, not every visible detail. The same picture can need different alt text in different places, because the surrounding words and the reason for including the image change what a reader needs to know.
Begin with the image’s job
Before writing, ask: What would a reader miss if this image were unavailable? If it shows a step, explain the step. If it labels a control, name its function. If it adds no information and serves only as visual decoration, it may not need alt text. W3C’s writing guidance makes this distinction: alternatives should provide the information or function of an image, while purely decorative images can be omitted. [1]
Suppose a phone-charging guide includes a close-up of a cable being inserted into the phone’s bottom edge. If the surrounding text only says “Connect the cable,” an alternative such as “Plug the cable into the phone’s bottom edge” supplies the location shown. A vague label like “Phone” does not. If the text already gives that exact instruction, however, repeating it may add little. Check the image beside its surrounding words.
Describe what a reader needs
Keep the description focused. Do not begin with phrases like “image of” or “picture showing” when the alternative is already attached to an image; the reading context identifies it as an image. Include a person, object, location, or visual relationship only when that detail helps explain the image’s purpose. Concision is useful, but there is no single ideal length for every image: a complex diagram may need a longer explanation elsewhere.
Consider an example from a community event page. A photo of a speaker on a stage may be decorative if the page already names the speaker and the image adds atmosphere. If the photo documents a sign language interpreter positioned beside the speaker, that relationship might be relevant to a description in a report about event access. The image itself has not changed; the purpose and surrounding text have.
Separate image text from image description
If an image contains words essential to the message, make those words available as text in the document when possible. A flyer image might show “Gate opens at 6:30,” but a text alternative limited to “event flyer” would hide the time. The alternative could convey the relevant wording, or the document could reproduce the date, time, and location as ordinary text. The best choice depends on the image’s role and what the surrounding content already supplies.
For a chart or detailed illustration, a short alternative can identify its topic, while nearby text or a longer description explains its important relationships and findings. Do not force every data point into a tiny field. Make sure the longer explanation is available in the document, rather than assuming the reader can infer the chart from its title.
Avoid inventory and guesswork
Alt text is not a caption for every visible object. Listing a blue sky, three chairs, and a window may distract if the image’s purpose is to show where a safety exit sits. Describe the exit’s location and relevant route instead. Likewise, do not guess someone’s identity, feelings, or purpose from appearance. State what can be supported by the image and what matters to the content.
When an image is linked, the alternative should usually communicate the link’s function or destination, not merely describe the picture. If a magnifying glass icon opens search, “Search” may be more useful than “magnifying glass.” If an image is the only content in a link to a larger map, convey the destination or function so that a reader can choose whether to follow it. [1]
Work through one image
Try this exercise with an image of a marked building entrance in a visitor guide. First, write the purpose: help a visitor find the step-free entrance. Then identify what the image adds beyond the text: it shows a side door beside the loading bay. A useful example alternative might be: “Step-free entrance is the side door beside the loading bay.” If nearby text already gives that route, the alternative could instead identify another useful visual distinction or be empty if the image repeats the same information and has no separate function.
Finally, check the result against the actual page. Does it duplicate a caption? Does it give a useful action or fact? Could a reader act on it without seeing the image? W3C recommends alternatives that provide the image’s information or function and says decorative images need no alternative text. [1] Those questions help translate that guidance into a decision for each image.
