Locate the option to add alt text to an image by going to the Picture Format tab in the toolbar and selecting Alt Text. Alternatively, you can right click on the image and select View Alt Text.

In this example from Microsoft Word 365, an alt text description was added by selecting the image and going to the Alt Text picture formatting option in the toolbar. The brief alt text was written in the corresponding text box and reads, "An underlined hyperlink in blue font reads, "Click here!" and has a red circle with a line through it in the front of it."

For information about writing good alt text, visit: 

In Microsoft Word 2019 and later, you may add "empty" or "null" alt text to an image that is purely decorative and does not communicate any information. This will allow a screen reader to skip over the image for a user. 

To add null alt text, open the alt text formatting option for the image and check the box at the bottom of the box labeled “Mark as decorative.”