Original image is removed when you add new draft and discard draft. No problem if you edit/discard already published translation.
The attached patch fixes the bug. The problem is a flaw in the query that checks if an image is used by several versions of an attribute.
1. Create and publish an object with an image (ezimage datatype, must be translateable)
2. Edit the object with new translation, set "Select the language the added translation will be based on:" to something other than "None"
3. Discard the draft
4. The original image file is removed. Other image aliases may still exist.