The same photograph can perform different jobs on two pages. It may establish atmosphere in one and demonstrate a product detail in another. I therefore treat alternative text as a contextual editorial decision rather than a permanent label attached to a file.
Identify the image’s purpose
A photograph, chart, linked icon and decorative texture do not require identical treatment. What information would someone miss without seeing the image? That question guides my first draft.
The W3C alt-text decision tree is a useful reference for considering information and function. An empty alternative can be appropriate for a purely decorative image; meaningful content needs a different assessment.
Compare two descriptions
Imagine a product page showing the inside of a bag. Beautiful black bag photograph names an object but misses the image’s purpose. A description identifying a zipped pocket and two open compartments may communicate the relevant detail more effectively.
If the same photograph is merely decorative within a cover collage, its role changes. Writing a long description for every use does not automatically improve accessibility. Unnecessary repetition can make listening harder.
A chart may need more than one sentence
When a chart carries important results, a short alternative may need to be accompanied by a textual explanation or data table. Naming the main trend does not replace access to the underlying detail. That requirement belongs in content planning.
If words embedded in an image already appear completely in nearby text, I consider whether repeating them helps. When the image is a functional link, its action matters more than a detailed account of its appearance.
Do not use the field as keyword storage
Unrelated search phrases do not serve someone trying to understand the image. I read the description aloud within the page and ask what it adds.
I also examine the mobile presentation and crop. If the text describes a detail that has disappeared from view, the asset may be correct while its use is not. The image and its explanation need to be managed together.