What to do
Include alternative text (alt text) with all visuals.
Visual content includes pictures, SmartArt graphics, shapes, groups, charts, embedded objects, ink, and videos.
Images that do not convey any information can be marked as 'decorative'.
In PowerPoint avoid placing text over images so it is visible to all.
How to find it
To find any missing alt text, use the Accessibility Checker.
Why do it
Alt text is used to provide an alternative description of the image for those people who are unable to view it. This may include people with visual impairments who use screen readers a well as people using text only browsers.
Avoid using text in images as the sole method of conveying important information. If you must use an image with text in it, repeat that text in the document.
How to do it
For any guidance regarding alt text, please have a look at this Cribsheet that tells everything in detail.
For more information and resources please visit Microsoft Accessibility Video Training.