Presentation Images: An Exception to Alternative Text
All images in your eBook should have a text alternative (EPUB Accessibility Techniques 1.0, 2017). The exception to this is if you have an image in your book that is strictly presentational. However, the absence of alternative text alone is not enough to inform assistive technologies that the image is presentation.
Presentation images in Thomas Becket, by Frank Barlow, published by the Folio Society, 2002
Use the following code where you want to insert the image.
<img src="/example.png" role="presentation" alt="">
The role="presentation" is an ARIA (Accessible Rich Internet Applications) role. This role will not hide the image to assistive technologies, but will enable them to know that they can safely ignore it.
For more information about the presentation role, see WAI-ARIA 1.1