After adding that definition, you need to force your .htaccess file to be regenerated. There are a couple ways to do this:

• De/Re-activate Role Scoper
• Go to Settings > Permalinks and re-save the current settings

If you've done that (and confirm that the .htaccess in your WordPress folder no longer has any mention of scoper_rewrite), attachment file filtering is out of the picture.

If the posts you're trying to pull images from are not readable to the logged user then yes, they will be blocked - either to nothing or to the teaser message. To get around this, you can include the argument suppress_filters=1 in your custom query.