If you use a Mac, click this link to go to the PowerPoint 2016 for Mac module.
Besides text, many PowerPoint presentations also contain images. The overall purpose of including images in your presentation should be to help the audience comprehend the purpose and/or meaning of the content. Some of the most common images used within presentations are:
Screen-reading software can only read text, not images or non-text graphics. Therefore, a reader who cannot see your presentation will only learn that there is an image there, not what the image is or what function it serves in the presentation.
Adding an alternative text description, also known as alt text, to the image alerts the screen-reading software that there is text to read aloud. When the screen reader passes over the visual element, it will read the alt text.
Alt text also makes web pages more accessible to anyone who is browsing on a weak internet connection or a small portable device like a smart phone. The alt text labels become visible while images are loading.
Alt text should not repeat what is already stated in captions or other presentation text. Think of it as a secret code that only screen reader users will be aware of. It will not appear to the naked eye, but rather is part of the presentation's code. Consider the following example:
Caption: In this painting the artist Emanuel Leutze used light and color to convey motion.
Notice that the caption does not really describe WHAT the image is. Add in alt text to give a description of what we see here. Maybe something like:
"George Washington looks forward with a determined gaze as his wary troops row their small craft across the Delaware River and into battle. The light of dawn, swirling water, and jagged icebergs add a dramatic effect."
Here is a video to show how to add alt text to a document in Word for Windows (PC). The procedure for adding alt text to an image in PowerPoint is virtually the same.