Deleting images after including them in an email will remove the image from the reader's email as well.
Assets within your media library are hosted online by Workshop. When a recipient opens an email with an image, the image must be present in the hosted location for the recipient to view the image.
If assets from the media library have been included in an email or template and then are later deleted, they will not display as intended and will appear as a broken link in your message.
Deleting Images in your Media Library
From your Media Library, you have the ability to delete individual files (including image files) or bulk delete.
If you select to delete, Workshop will display a confirmation modal that alerts you to the impacts of deleting your file, as seen here:
This message will show to you for both actions - individual & bulk delete.
Best Practices
If you have any images that have been used in previous emails but no longer need to be included in media library, create an "Archive" folder and store unnecessary images there.