This Subscriber Downloads add-on let’s you obfuscate any link to any file (PDF, image, etc) in your media library. Requires user to have a valid Leaky Paywall subscription before downloading content.
How to restrict a file to subscriber views only
After installation create a link to your file with this format:
media_id is the ID of the attachment (found in the media section of WP). You can find the ID in the urlof the image while viewing it in the media library. See below: