A photograph of a cable connector, a bar chart, and a decorative landscape need different descriptions. What matters is the information each image contributes to the document. An image description should make that contribution understandable when the picture cannot be seen. It is neither a filename nor automatically an exhaustive inventory. The surrounding text helps determine which details matter and which would distract.
Work from the image's purpose
Ask what understanding would be lost if the image disappeared. In a connection guide, it might be the location of a socket. An appropriate description could identify the highlighted connector immediately to the right of the power input. The color of the desk probably contributes nothing to that task.
For a chart, connect its message with the relevant quantities. “Bar chart” alone tells readers nothing about the values. Describe the main pattern and provide required figures in accessible text or a table. If readers need to compare exact numbers, a short summary in alt text is not enough.
Avoid unsupported interpretations of people or situations. You can describe a visible action, but feelings, identities, and intentions often cannot be inferred from the picture. Likewise, describe technical characteristics of a pictured device only when they are visible or established by reliable information.
A purely decorative image can be marked as decorative in applications that provide that option. Make that decision deliberately. A graphic containing a warning or important text is not decorative simply because it looks attractive. If a screenshot contains essential operating instructions, those instructions also need to be available.
Coordinate alt text, captions, and surrounding prose
Enter the description in the application's alt-text field. A visible caption and alt text have related but different purposes. A caption may provide a figure number or source; the description explains the image's contribution for someone who cannot see it. Repeating the same wording unnecessarily can make reading tedious.
Automatically generated descriptions are drafts, not verified statements. Check numbers, text within images, positions, and technical terms. If the automatic description says only “computer on a desk” while the illustration explains a particular connector location, it has missed the essential point.
Read the description alongside the paragraphs before and after the image. Is it understandable without looking at the picture? Does it need a direction such as “on the left of the image,” or is the location already clear? Do not work to an arbitrary character count: a simple photo may need little explanation, while a complex graphic may require additional discussion in the document.
Finally, check the delivered format. Conversion may lose image descriptions or structural information, so inspect the actual file being shared and use the integrated accessibility checker as another aid. For an important document, feedback from people who use assistive technology can be particularly informative; an automated test does not replace that experience.