Identify the image’s job
Before writing alternative text, ask why the image is on the page. Does it communicate information, perform a function, or add decoration beside text that already supplies the meaning? W3C’s Images Tutorial organizes guidance around these different purposes. The same photograph can need different treatment in different contexts.
Imagine a neutral desk photograph on a guide about preparing files. If it only sets a mood, a long inventory of objects may distract from the instructions. If a marked-up screenshot identifies a missing link in a document, the relevant information concerns that missing link and how the reader can identify it.
Write the useful meaning
For an informative image, write an alternative that conveys what the reader needs from it in this setting. Avoid adding a promotional interpretation that the image does not establish. A room photograph does not prove a company’s experience, a product’s quality or a health outcome, even when those ideas suit the surrounding brand story.
Use the nearby text to avoid unnecessary repetition. If a caption already explains part of the information, decide what additional meaning the image contributes and how it should be available. Keep the alternative in the content record with the image so it can be reviewed when either the image or surrounding copy changes.
Describe a function as a function
When an image is used as a link or control, its accessible alternative needs to communicate the action or destination. A linked logo that returns to a homepage has a different job from an unlinked logo used for identification. W3C provides functional-image guidance for this distinction.
Review the complete control, including visible text and any other accessible label, rather than adding competing descriptions independently. Ask a collaborator to check the actual implementation. A planning document can specify the intended function, but it cannot establish that the final control exposes that function correctly to assistive technology.
Handle decoration and complex information deliberately
Decorative images may use an empty alternative in the appropriate web implementation; this is a deliberate treatment, not a missing decision. Document that purpose so a later editor does not replace the empty value with an unhelpful description. Check that the image really adds no necessary information before choosing this approach.
Charts, diagrams and detailed screenshots may need information beyond a short alternative. Put the relevant explanation or data in suitable nearby content rather than packing every detail into one sentence. W3C’s Images of Text guidance also helps identify words that should remain actual text instead of being available only inside a graphic.
Review the finished context
Read the page with the image hidden, then ask whether its meaning and functions are still available. This is a useful editorial check, not a complete accessibility test. Revisit the alternative when the image, link destination or article changes, and keep implementation questions with the person responsible for the final format.