Provide Alt text for each image

For Office documents, you can add text alternatives by right-clicking the image and choosing ‘Edit Alt text’, or by selecting the image and clicking the ‘Alt Text’ icon on the ‘Picture Format’ ribbon.

Techniques

If the image is purely decorative then mark it as decorative

For Office documents, you can specify decorative images by checking the “Mark as decorative” checkbox in the Alt Text pane. Note: although there will be no text in the textbox, it is essential that you check this checkbox as this communicates the image’s decorative nature to screen readers

For all other instances, use alt text that accurately describes the content of the image.

Type the Alt text into the box in the Alt Text pane;

Alt text is always dependent on the context of the image and the available text around it. There is a very good illustration of this on WebAIM’s alternative text page
Read CP 1.3.1 - All images must have a text equivalent for further guidance on Alt Text.

Examples of good practice

The "Alt Text" pane with Alt Text describing the nature of the image — Figure 1

The "Alt Text" pane with "Mark as decorative" checked so that its decorative nature can be accurately conveyed to screen readers. — Figure 2

An example of bad practice

The "Alt Text" pane with no Alt Text supplied and the "Mark as decorative" checkbox unchecked — Figure 3

References

WCAG 2.1

1.1.1 Non-Text Content (A)

EN 301 549 v 2.1.2

9.1.1.1 Non-Text Content

Further reading