The same photograph can need different alternative text on two pages. In a walking guide, it might show an important feature of a path. In a decorative banner, it might add atmosphere without adding information. Start with the image’s role in the current page, not a list of everything visible in the frame.
The W3C WAI alt decision tree distinguishes informative, functional, decorative and other image uses. It is a practical starting point for deciding what a reader needs from an image alternative.
Ask what would be lost without the image
If the image explains a detail the article depends on, describe that detail. In an illustrative discussion of a damaged cable, Frayed outer insulation near the connector may be more useful than Black cable on a white table. Do not claim details you cannot see. The purpose is to preserve useful meaning, not to make every photograph sound dramatic.
If surrounding text already provides all the information and the image is purely decorative, an empty alt attribute may be appropriate. Empty alt text and a missing alt attribute are different implementations. Check that your publishing system produces the intended result rather than relying on what the editing field looks like.
Describe the action when the image is a control
An image used as the only content of a link or button needs an alternative that communicates the destination or function. A magnifying-glass icon that submits a search should be labelled for the action, not merely described as a drawing. If visible text already names the control, inspect the combined accessible name to avoid unnecessary repetition.
Give complex information more space
A chart with several trends usually needs more than a short alt phrase. Provide the important information in nearby prose or an accessible table, and use the brief alternative to identify the chart. Do not compress a whole spreadsheet into an unreadable sentence. Publish meaningful words as real text wherever possible so readers can select, enlarge and translate them.
Read the paragraph before the image, the alternative and the paragraph after it. Does the sequence make sense? Remove repeated captions, irrelevant detail and keyword lists that do not help understanding. If a generated picture is an illustrative scene rather than evidence of an event, explain that in a visible caption. Alt text describes the image; editorial context explains how it is being used.