This is odd. I have permissions set up for guests to view attachments. It appears that as a guest, you can view the original attachment. However, when the image is quoted, there is a 403 error when hitting the link?
The quoted image reference is not the attachment in the post. I see this is an old post, so this is likely related to the import. When importing, embedded attachment references are only updated in the post with the attachment; it's not really viable to handle it in quotes.