The page a shared image link opens (the share mini page) links the article the image was shared from when the option Share article links is on. It took the address of the article from the shared link and printed it into the page without checking or escaping it, and with the link type Image Page with Redirect it followed the address at once. A prepared link could therefore run a script in the page, in the session of the visitor who opened it, or send the visitor to another web site. Nothing on the server is changed or read by the server.