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.
Right-click an image on the Power Point slide. Select “Format Picture.”
In the pane that pops up to the right of the slide, select the Size & Properties tab, and then click the Alt Text dropdown.
Type a description of the image in the description box — this text will serve as the image’s alt text for both the Power Point file and the resulting PDF if you create one. 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.