Alternative text content must be provided for all non-textual content (most commonly images).
Unfortunately, there's no quick fix for images that are already published without alternate text -the best way to avoid getting to this point is to be sure to add alternate text at the point of inserting your image into the page.
In order to fix published images which do not have alternative text, you'll need to
check-out the pages in question, click on each image missing alt text, go to "Insert/Edit
Image" in the toolbar and type some alternate text in the alt text field. TIP: The best way to compose meaningful alt text is to imagine you are trying to describe
the image to someone over the phone.