How can you delete images in Photo Center that have existing derivatives?
The best solution is to search for a certain metadata field that you know is the same in the Photo Center search, for example, the caption field.
That search provides a list of all derivatives, and you can then delete the derivatives in order, from the last derivative created, up to the original image.
Note
When you delete an image from Photo Center, the system retains references to that image anywhere it appears in your articles. The image reference is broken, so you must manually delete those references from your articles.