Perhaps you have all the images you want in your book on another website already, or are producing a book on Pressbooks.com and have already met the media storage limit for your upgrade.
It is technically possible to hotlink images to sources outside of Pressbooks. The process is similar to how videos work in Pressbooks; the content is hosted on another website and server, but displays in your book without having to be added to the Media Library.
This method of adding images to your book works fine for the webbook format, though please note that any image which is hotlinked from outside of Pressbooks may change if the image or its settings are altered on the host website. You'll also have fewer editing options in Pressbooks for any image which is not hosted in your media library.
However, we do caution that this is not the best method if you are creating exported file formats for publication.
EPUB files which do not contain the original source files of all media will not validate per the global standards of the IDPF, and therefore may not be accepted to ebookstores.
Though hotlinked images won't cause any validation errors in your PDF files, you may also run into issues if your printer requires all media to be packaged with the file.