Add alt text to images, such as pictures, clip art, and screenshots, so that screen readers can read the text to describe the image to users who can’t see the image.
To add alternative text to an image in Word for Windows:
Right-click an image.
Select Edit Alt Text.
Type a description and a title.
TIP: Include the most important information in the first line, and be as concise as possible.
For more hints and guidelines for writing good alt text, visit the page Writing for Accessibility: Alt Tags and Long Descriptions.