If a user changed an album for i.e. "owner only" and a picture from this album is embed in a post, it is visible for everyone, for guests, too.
Is it a bug? I think, if a user hide his pictures he do not want to show it in the posts.
When you edit that post, is the image in IMG tags or GALLERY tags?
If it is the path to the thumbnail in an IMG tag then this is expected. Thumbnails don't have any permission checks on them. And this is basically the same as attachments (if the thumbnail path was embedded in a post rather than an attach tag).