Alternative text, the text equivalent for an image. It is read by screen readers allowing the content of the image to be accessible to those with visual or certain cognitive disabilities.

Graphics in courses fall into two broad categories: images that convey course content and those that don't. Focus on those that do. Decide which images represent information significant to the content. Assign alt text to these images. Balance your decisions with a recognition of the additional time it will take your visually impaired learner to scan through the page. For example, alt text that describes the physical characteristics of your course avatar may have significance for some topics but not for others.

Leave the alt text empty ("alt": "") if the image does not contribute significant course content. If the alt text is left empty, the image will not be included in the tab order.

• Be specific and succinct. Describe what you see. A few words will usually be enough, but charts and diagrams will likely require more thought. Bear in mind that screen readers may cut off alt text at around 125 characters.
• Don’t be redundant. If an image doesn't add value and is just there for design purposes, or the context is already covered in the supporting text, leave the alt text empty.
• Don’t use phrases “image of…” or “graphic of…” etc. It's already assumed your alt text is referring to an image, so there's no need to specify it.

• https://webaim.org/techniques/alttext/