When you add an Image to a Page or Post, the automatically gets attached to that Page or Post.
An Image can be displayed on several Pages or Posts but can be attached to only one.
You can Add an Image to your Media Library or to a Page or Post directly.
- Add to Media Library: image remains unattached until you use it in a Page or expressly attach it to one
- Add to Page or Post: image is attached to that Page ID, and automatically added to your Media Library