Images in PowerPoint
It is very common to use images on PowerPoint slides. They help to improve the look and feel of a slide, as well as giving additional information to support the message. When you insert an image or any other non-text content on a slide, you must give it alt text. This relates to WCAG 1.1.1 which states:
All non-text content that is presented to the user has a text alternative that serves the equivalent purpose...
The following videos give more detailed instructions about how to add alt text and how it works.
Basic alt text
Basic alt text should be a brief description of the image or non-text item. Ideally, it should be less than 150 characters long.
Complex alt text
If you need more than 150 characters to describe the image, then you may need to consider giving a long description as well.
How screen readers read alt text
Screen readers read alt text in a slightly different way to how they read regular text. With regular text, the user can read the whole block of text, word-by-word, or even character-by-character. This means that if the user doesn't understand something, they can explore it to find out how it is spelled. Alt text doesn't allow the user to do this. They can hear all of it or skip it completely. So try to keep alt text brief and simple.