You can add illustrations from online sources to Knowledge Maps. These are featured image files that are representative of the place or subject. You can add any file accessible via URL in the "Illustrations" section of the KMap editor. If your image is not already hosted online, you can add it to Images in Mandala.
You might want to use Mandala Images as illustrations. In this case, you should use the URL of the image, which will look something like https://cicada.shanti.virginia.edu/images/mandala/shanti-image-617461/full/!1000,/0/default.jpg?1582174694691. (This is not the same as the URL to the image and associated metadata, which will look something like https://images.shanti.virginia.edu/image/untitled-57489.) To get this URL:
The exact wording of this will depend on your browser; you might see an option like Copy Image Address instead
If you use Copy Image Address, the image URL will copy to your clipboard automatically; you can then use that URL in KMaps by following the guide above