If you give an external user Read permission to access the Organization Asset library , then the external user can use the assets inside the Organization Asset library.
#Create an organization assets library
Add-SPOOrgAssetsLibrary -LibraryUrl <URL> [-ThumbnailUrl <URL>] [-OrgAssetType <ImageDocumentLibrary or OfficeTemplateLibrary>] [-CdnType <Public or Private>]
- When Public CDN is used for asset library, all content in the
library is anonymously available for anyone having the URL.
Public CDN should be used for non-sensitive assets, like images,
JavaScripts and css-files. Private CDN respects SharePoint library
permissions. So if user doesn’t have access to original library,
content is not displayed. This also considers external users.
- If you are planning to use only Private CDN configured asset
libraries, you need to make sure that users consuming the assets have
access to original libraries. The simplest option is grant everyone
a read permission to site hosting asset libraries.
More information, please refer to this article.
======================== Updated Answer ======================
If you choose to use Public CDN, then all content in the library is anonymously available for anyone having the URL. Public CDN should be used for non-sensitive assets, like images, JavaScripts and css-files.
Private CDN respects SharePoint library permissions, so it can only be accessed by users with permissions to the original document library or storage location. So if user doesn’t have access to original library, content is not displayed. This also considers external users.