Alternative text is a text replacement for an image, provided through the img element's alt attribute. In Joomla content, it conveys meaning when visitors cannot see the image or use a screen reader.
Choosing the text
Describe informative images according to their role in the article. For a settings screenshot, identify the relevant control and meaningful state. Avoid listing every detail or needlessly repeating nearby text.
Use an empty alt for purely decorative images. If an image is the sole content of a link or button, describe the action or destination. Complex diagrams may need a longer explanation nearby.
Alt in Joomla
The Joomla editor provides alternative text for individual images. A title attribute, filename or visible caption is not automatically a substitute for alt.
After saving Joomla content, inspect the final img and its context because templates and extensions may generate images differently. Translate alternatives for the English Joomla version according to meaning. A keyword list is not a useful alternative.
Conclusion
Good alt text conveys the necessary meaning. Its wording depends on the image function on that page.