Image thumbnails in parent post for unpublished images

Issue Summary

If a generated image node is unpublished the thumbnail for the image is still displayed on the parent post. When clicked on the (correct) Access Denied error is generated for the actual image node (unless you are using Lightbox2 - in that case the full size image is displayed as normal).

Marking as critical because this bug allows users to see content (the thumbnail) that may have been un-published for reasons such as inappropriateness.