Two identical images on different pages may need different text alternatives. A chair photograph in a catalogue presents a product; a small copy beside an already named link may simply repeat its meaning. Asking “what is pictured?” is useful, but insufficient. First establish why the image is here and what someone who cannot see it would miss.
Separate three jobs
An ordinary informative illustration needs a concise alternative conveying what matters in context. For an image that is the only content of a button or link, the action or destination matters more. Pure decoration should not create unnecessary reading. W3C’s decision tree helps distinguish these situations, but does not replace editorial understanding of the material.
Describe what people need to notice
An imaginary room-planning article might show a chair beside a passageway. Its position matters here, rather than every shade of upholstery. Do not add properties the photograph cannot establish: comfort, quality, effectiveness or being “the best”. A conceptual illustration can be identified as an illustration. A visible caption can separately explain the material’s origin and details useful to everyone.
Name the action when an image is a control
A magnifying-glass icon might open search, enlarge a photograph or reveal details. Describing its shape does not explain what activation will do. When the same link already contains a complete text label for the action, the image may be redundant. Check the accessible name of the whole control rather than giving every picture the same phrase.
Empty alt is not missing alt
For a decorative image, W3C describes an empty text alternative. This is a deliberate choice, different from omitting the attribute. Do not use it to hide important information. If a diagram contains several relationships or a chart conveys data, a short alternative cannot replace a complete explanation: provide the information in accessible nearby text.
Read the page as a conversation
Read the text and image alternatives together in page order. Does one name repeat three times? Can you understand what a button does? Is any information available only inside a picture? Then test actual reading with assistive technology. For another language, translate meaning and action, not the filename. Good alt text helps explain the page; it is not a space for promotional lists and does not guarantee search rankings.