For technical reasons we generally advise against embedding images as base64 data directly within a Twine project.
The linked Twine Cookbook recipe does include an example of using a HTML <img> element to display an image that is stored/hosted on web-site, using an Absolute URL as the src attribute value.
And Jasper’s comment in the I would like to display images in twine thread of this forum shows an example of using a Relative URL as the src of a <img> element.